I don't guarantee that following this guide is the absolute most efficient way to gain affection points throughout the game, but it'll get you more than enough affection points with every character to qualify for the harem ending. This guide was prepared using a beta version of the game, so it's possible that there may be some slight changes in affection point gains in the final version, but there's enough leeway that you should be fine regardless.

Some characters are easier to raise affection with than others. Cici in particular has a LOT of affection points you can pick up by playing her video game if you make the correct choices. When choosing between affection gains for different characters, it generally makes sense to prioritize the ones who are more difficult to raise, which is what this guide tries to do.

Note that if you want a specific character's ending, you'll have to do all of that character's personal story arc scenes in addition to meeting affection point requirements. The harem ending doesn't require seeing any specific events and simply requires a high (60+) affection value for every single character. I'll note missable arc scenes where they occur.

Affection point gains in dungeon subevent scenes are usually 3 points per character; gains in story events are usually 1-2 points.

At the very end of the game, you'll be able to choose freely from a list of every ending that you qualify for. This guide is designed to unlock as many endings as possible on a first playthrough, which means the harem ending plus every individual character ending except Kairi's. There's an event early in the game where you can see scenes with Jordan, Summer, Kairi, and Cici that are all required for their individual endings, but you only have time to see three of them. There's also an event right near the end of the game with mutually exclusive scenes for Kairi and Pearl that are required for their endings. This means that if you neglect Kairi for both of those events, then you can get every other ending. If you want to see Kairi's ending, you'll have to lock yourself out of Pearl's ending and one of Jordan's, Summer's, and Cici's. I've noted the points where you'll need to choose differently in order to unlock Kairi's ending.

Opening cutscene:
How? This is a dream. (+Pearl)
You look the same. (+Pearl)
I'll protect you, Pearl! (none of the choices here have an affection gain, but the third choice loops back and makes you pick again)
Let me eat first... (+Pearl)

Police station intro:
First choice doesn't matter
So obviously, you mean... (+Jordan)
Examine the PC and open up happybdayjo.docx (all of the other options are just flavour text with no affection point gains; you can see them or skip them as you wish)
Let's mosey.
You and me, alone? (+Jordan)

Affection points at start of police station dungeon: Pearl 7, Jordan 5

Police station dungeon:
1F x2 y4: Rein her in, Jordan! (+Jordan)
2F x1 y8: It's more awesomer. (+Pearl +Jordan)

Police station story event 1 (2F x4 y6):
Check Yourself (+Jordan), then go through all the other options and leave. This unlocks the eastern half of the dungeon.

Police station dungeon, continued:
1F x25 y23: Good times, horsing around. (+Pearl)
1F x28 y10: Pearl. Intervention time. (+Pearl)
2F x23 y14: No way she's this cute! (+Jordan)

Police station story event 2 (2F x20 y20):
Make sure Jordan and Pearl both have at least 10 AFF before triggering this event. If you've been following the guide, they should both have 16.

Mall intro:
First two choices don't matter
After the intro, you'll be able to access Pearl and Jordan's first personal arc scenes if you have at least 10 AFF with them.

Pearl arc scene 1:
You looked after me all along? (+Pearl)
I'm relying on you. (+Pearl)

Jordan arc scene 1:
So, that movie we saw last... (+Jordan)
Make a boob joke (+Jordan)

After completing both scenes, choose "Just Walk Away" to move on to Kairi's intro scene.

Kairi intro:
First choice doesn't matter
Are you a science prodigy? (+Kairi)
Check the newspaper rack (+Kairi) and then pick any of the other three options to continue
Next choice doesn't matter: you now have Kairi in your party and access to the mall dungeon

Note that from this point on, dungeon subevents will require you to have specific characters in your party. You'll recruit two new characters, Summer and Cici, as you progress through this dungeon. Start with the eastern part of the mall, since it's easiest.

Mall dungeon:
East x17 y5 (Pearl/Summer): Bomb's away! (+Summer)
East x1 y5 (Cici/Pearl/Kairi): Scarred is sexy. (+Cici +Pearl +Kairi)
East x6 y15 (Pearl/Kairi/Jordan): Ate all the jerky, huh? (+Kairi)
East x14 y28 (Jordan/Cici): True. I'm here! (+Cici)
East x22 y27 (Pearl/Jordan): Zombies are gross. (+Pearl)
East x18 y17 (Summer/Cici/Jordan): That means no motor oil. (+Jordan +Cici)

Mall dungeon story event 1 (East x7 y6):
Meet Summer. The first choice doesn't matter, but the sparring match does.
When she grins aggressively, block.
When she swats at her nose, feint.
When she shakes her hands, feint.
When she seems to be waiting for something, attack.
When you sense zero nonsense from her, counter.
The fight is best 3 out of 5, and the best possible affection gain comes from dragging it out as long as possible while still winning, so you should lose two rounds on purpose and win the rest.
The choice after the fight doesn't matter. Summer has now joined your party.

West x21 y3 (Jordan/Cici): Cici, pack 'em. (+Cici)
West x3 y12 (Pearl/Cici): Is there a problem? (+Cici)
West x21 y25 (Jordan/Pearl/Summer): Pearl's best dish is... (+Jordan +Pearl)
West x21 y9 (Summer/Cici): See the forest, not the trees. (+Summer)
West x2 y21 (Pearl/Jordan/Kairi): Kairi's bags (+Kairi)

Mall dungeon story event 2 (West x7 y25):
I can attest to that. (+Cici)
Cici joins your party after this event. Her starting affection is pretty low but she'll have many opportunities to gain more.

North x12 y13 (Pearl/Cici/Summer): Liar. (+Summer)
North x8 y3 (Cici/Jordan/Pearl): So, the motor oil... (+Cici)
North x4 y22 (Cici/Kairi): See? (+Kairi)
North x1 y28 (Kairi/Cici/Summer): Sad and amusing. (+Kairi +Cici)
North x19 y21 (Jordan/Summer/Kairi): Goodness gracious. (+Summer)

Mall dungeon story event 3 (North x12 y30):
If you've been following this guide, your affection levels at the end of this dungeon should be Pearl 27, Jordan 25, Kairi 22, Summer 21, Cici 26. You'll soon be coming up on another set of character scenes with affection requirements, so make sure you've done all the dungeon subevents before triggering this event.

After finishing the mall, you'll reach a roadblock on the way to your next destination, where you'll have time to see 3 of the available events. There are 4 different character arc scenes here, so this is the first point at which you'll have to lock yourself out of someone's ending. In this guide, we'll be skipping Kairi's.

Go after Cici:
Idiocy kills. (+Pearl)
Cici has a BIG backyard. (+Cici)
From that day, yes. (+Cici)
Lend an Ear (begins Cici arc scene; note that there are a lot of missable affection points in this scene and some options will end the scene abruptly, so follow the guide carefully)
Read Signpost
Check Cloth (+Cici)
Go West
Go West
Climb Ladder (+Cici)
Move Ladder
Take Cloth (+Cici)
Go South
Go South
Ask Boy
Smash Pot (+Cici)
Return North
Return North
Return East
Light Match
Eat Firefly (+Cici)
Check Bog
Read Signpost
Return North
Go West
Go South
Examine Gate (+Cici)
Go East
Check Door
Climb Stairs
Enter Door
Go North
Didn't peg you as an idiot. (+Cici)
Cici scene ends, return to roadblock. You just got a total of 11 affection points with Cici and 2 with Pearl. See what I mean about her scenes throwing a lot of free points at you?

Hit up the Store:
What's the plan, Jordan? (+Jordan)
The free radicals of food! (+Jordan)
This is trash, right? (+Summer)
Summer! (+Summer)
Faux-Ignore Her (unlocks Jordan's arc scene)
Your memory... (+Jordan)
I'm more of a Flattop. (+Jordan)
Jordan's scene ends, return to roadblock.

Join the Hunt:
Yep, in the face. (+Summer)
One other thing first. (+Summer)
You're on! (unlock's Summer's arc scene)
The fight works the same way as before, so drag it out to 5 rounds but make sure to win
Nah. You've got the touch. (+Summer)
Softball (+Summer)
I'll take the bird. (+Summer)
Summer's scene ends, proceed to Krik Elemental Labs intro.

Lab intro:
Cici found her weakness. (+Jordan +Kairi)
Is there an entrance mat? (+Kairi)
Unlock government building hub area

Your ultimate goal in this area is to get into the labs, but right now you have access to the school and hospital dungeons. Definitely do the school first, as the hospital is much more difficult. Your affection levels after finishing the roadblock scenes should be Pearl 29, Jordan 35, Kairi 26, Summer 36, Cici 37. Since we'll be missing Kairi's personal arc scenes and the associated affection points this playthrough, we'll need to focus on raising her points a little more in dungeon subevents.

School story event 1 / Emily arc scene 1 (2F-C x23 y17):
No. (+Emily)
No. (+Kairi +Emily)
Nah. (+Summer +Kairi)
Yes. (+Pearl +Emily)
Huh...? (+Kairi +Jordan)
I don't answer to you! (+Emily)
Talking to others? (+Emily)
To find you. (+Emily)
You need us. (+Emily)
Junk. (+Emily)
You. (+Emily)
Last choice doesn't matter (+Emily for any option)
Emily has now joined your party. If you followed the guide she'll start with 14 affection, but her affection requirements for future scenes are a little lower than the other characters to make up for the fact that she joins later.

School dungeon subevents:
1F-C x21 y10 (Pearl/Kairi/Cici): Well, statistically speaking... (+Pearl +Kairi)
1F-C x42 y8 (Summer/Pearl/Kairi): Choice doesn't matter (+Pearl either way)
1F-C x47 y3 (Summer/Pearl/Jordan): On the contrary! (+Summer +Pearl)
1F-C x0 y9 (Jordan/Cici): Private school uniforms... (+Jordan)
1F-C x9 y20 (Jordan/Summer): She's a private person. (+Jordan)

1F-E x21 y3 (Summer/Kairi/Cici): It was okay. (+Summer +Kairi +Cici)
1F-E x26 y7 (Summer/Jordan/Emily): Not yet, anyway. (+Summer +Jordan)
1F-E x24 y22 (Cici/Summer): Cici, you're a riot. (+Cici)

1F-W x5 y4 (Pearl/Emily): Roll with it, Pearl. (+Emily)

2F-C x32 y3 (Jordan/Cici): Street Sharks (+Jordan)
2F-C x1 y11 (Kairi/Emily): Dichroma is a weapon! (+Emily)
2F-C x11 y2 (Summer/Cici): So... do you? (+Summer)

2F-E x15 y2 (Summer/Kairi/Pearl): Knocked down? Get up. (+Summer +Kairi)
2F-E x16 y16 (Cici/Kairi): Tousle her hair (+Cici +Kairi)
2F-E x23 y29 (Emily/Cici): She's too cuddly? (+Emily +Cici)
2F-E x9 y29 (Cici/Pearl/Jordan): Freedom! (+Pearl)

2F-W x2 y10 (Emily/Summer/Kairi): You're already dead. (+Kairi +Emily)
2F-W x21 y15 (Summer/Emily): Here's Dichroma. (+Emily)
2F-W x8 y21 (Jordan/Emily): I'll spoil you!! (+Emily)

School story event 2 / Cici arc scene 2 (1F-E x16 y21):
Note that there's an affection requirement for Cici's arc scene, so make sure she has at least 40 before triggering this event.
A dead kid's journal... (+Kairi)
A school for gifted youngsters? (+Summer +Jordan)
Erase her (+Pearl +Cici +Kairi)
Bait her in! (Unlocks Cici's arc scene; once again, there are some options that will end this scene prematurely, so follow the guide carefully)
Go East
Go South
Explore Den
Examine Table (+Cici)
Step Back
Explore Bathroom
Examine Sink
Check Cabinet (+Cici)
Examine Shower (+Cici)
Exit Bathroom
Go South
Check Table (+Cici)
Go West
Check Juicer
Activate Burner
Low
Check Nuts
Return East
Return North
Explore Den
Click Lamp (+Cici)
Step Back
Return North
Return West
Check Bookshelf
Lil Bro, Big Heart (+Cici)
Go Back
Break Piggy (+Cici)
Go East
Go South
Check TV/VHS (+Cici)
Step Back
Go South
Go Outside
Proceed Outside
Cici scene ends, continue exploring the hospital.

School story event 3 (1F-W x4 y8):
Makers of Ex Body Spray. (+Emily)
Follow Pearl and Jordan
The Claw introduction. (+Jordan)
Who cares. (There'd be a Kairi scene here, but we locked ourselves out of it by missing her earlier scene at the roadblock)

You've now done everything you need to do at the school dungeon, so it's time to move on.

At any point after completing the school, you can visit the karaoke bar. You'll need to visit it at some point in order to proceed, but make sure you have at least 40 affection with Summer and 25 affection with Emily first in order to avoid missing subevents. That shouldn't be a problem if you're following this guide, but if you're not quite there yet, skip ahead to the hospital for now and come back here later.

Karaoke bar / Summer arc scene 2 / Emily arc scene 2:
My entrails are your red carpet. (+Cici +Emily)
You and Emily (+Emily)
Sentinels of the Abyss. (+Emily)
Bring It Up (unlocks Summer arc scene)
Oops. Hit a nerve? (+Summer)
Dump 'em. (+Summer)
So... what'd they do? (+Summer)
Give her a pep talk (+Summer)
Dibs on corpse party. (+Kairi +Summer)
Be Brave! (+Emily, unlocks Emily arc scene)
Why keep it secret? (+Emily)
Do you even taste it? (+Emily)
So about that place... (+Emily)

Hospital dungeon subevents:
1F-C x7 y8 (Pearl/Cici): What's going on? (+Pearl)
1F-C x18 y19 (Pearl/Kairi/Emily): We need offspring! (+Pearl +Kairi)
1F-C x10 y27 (Summer/Jordan/Emily): She knows all... (+Emily)
1F-C x16 y24 (Jordan/Kairi): I spit in her hair. (+Jordan)

1F-E x15 y2 (Kairi/Jordan): I love it here! (+Kairi)
1F-E x19 y17 (Summer/Cici): Masochist. (+Summer)

1F-W x20 y7 (Emily/Jordan): That's genius! (+Emily +Jordan)
1F-W x15 y8 (Kairi/Emily): Leave Dichroma alone! (+Emily)

2F-C x3 y4 (Kairi/Emily): You're plenty cute. (+Emily)
2F-C x23 y8 (Kairi/Summer/Pearl): Skin and Zombones (+Pearl)
2F-C x18 y28 (Cici/Kairi/Jordan): Ahhh yes!!! (+Cici +Kairi)

2F-W x6 y24 (Cici/Pearl): Follow your heart. (+Pearl)
2F-W x2 y13 (Cici/Emily/Summer): Blessed parents! (+Summer +Cici +Emily)
2F-W x26 y10 (Kairi/Cici): U-Um... (+Cici)

Hospital story event 1 (1F-E x23 y20):
Ladies first. (+Summer +Pearl)
Tornaaaadoooo! (+Kairi +Jordan)

Hospital story event 2 / Pearl arc scene 2: (1F-W x3 y13)
Make sure Pearl has at least 40 affection before triggering this scene.
Science had no idea... (+Pearl +Jordan +Kairi +Summer)
He turned me into an adult. (+Jordan +Kairi)
Loyalty! Honesty! (+Jordan +Summer +Pearl)
Let Her Speak (unlocks Pearl arc scene)
Let's start with what. (+Pearl)
Next choice doesn't matter
Embrace her (+Pearl)
Pearl scene complete, continue exploring the hospital

Hospital story event 3 (1F-W x4 y21):
No choices, but gives you some free points for Emily and is necessary to continue the game

You now have access to the lab dungeon. If you've been following this guide, your affection levels should now be Pearl 66, Jordan 65, Kairi 64, Summer 73, Cici 73, Emily 63. You already have enough affection on everybody to qualify for the harem ending, but there are still some scenes we'll need to see to qualify for individual character endings.

Lab dungeon subevents:
1F x4 y11 (Pearl/Kairi): Guilty. (+Kairi)
1F x15 y5 (Cici/Pearl/Jordan): Secret ingredient: Snails! (+Pearl)

2F x7 y9 (Jordan/Summer): Derivative works. (+Jordan)
2F x14 y6 (Pearl/Emily): It's true. (+Emily)

3F x11 y9 (Pearl/Jordan/Kairi): The hugs. (+Pearl +Jordan)
3F x7 y5 (Kairi/Summer): It was always dead. (+Kairi +Summer)

4F x14 y4 (Kairi/Emily/Pearl): Nothing much. (+Emily)
4F x2 y2 (Summer/Jordan/Kairi): Price gouging. (+Kairi)

5F x19 y3 (Cici/Jordan/Summer): May not be aborted! (+Cici +Jordan)
5F x13 y10 (Emily/Pearl): Let's find the cheese. (+Emily)

6F x9 y15 (Summer/Emily/Jordan): It happens. (+Summer)
6F x12 y5 (Emily/Cici/Kairi): No choices in this subevent (+Emily +Cici)

Lab story event 1 (7F x2 y3):
You must be THIS tall to ride. (+Cici +Emily)

Lab story event 2 (7F x2 y5):
Check the equipment (+Kairi +Jordan +Summer)

You'll now proceed to the train. There are 6 events here and there's only time to do 4. The Buck and Pearl/Kairi events aren't required for anyone's ending, so those are the ones we'll be skipping. The other four characters have arc scenes that you can see if you have sufficient affection (60-70 depending on the character) and are required for their individual endings.

Emily train event:
Dancing Dichroma? (+Emily)
I can't say. (+Emily)
But did you really...? (+Emily)

Jordan train event:
No thank you! (the other option ends the scene prematurely)
Be yourself. (+Jordan)
Blunt is best (+Jordan)
Zombified Hoarse Feed? (+Jordan)

Cici train event:
That pencil... (+Cici)
Promise to come clean. (+Cici)
Go East
Check Tree
Check Mailbox
Return West
Go West
Say Heeeey!
Give Disc (+Cici)
Return East
Go East
Check Tree with Reggie (+Cici)
That's normal. (+Cici)

Summer train event:
Give 110% (the other option ends the scene prematurely)
Boo-yah!! (+Summer)
It's kinda refreshing. (+Summer)
Feel better? (+Summer)
Don't be a fickle pickle. (+Summer)

If you've been following the guide, your affection levels when the train arrives at the sewers should be Pearl 72, Jordan 79, Kairi 75, Summer 79, Cici 85, Emily 85. At this point you easily have enough affection to qualify for every character's ending (except Kairi, since we skipped her arc scenes), although Pearl and Emily still have mandatory scenes ahead if you want to see their endings. Feel free to skip the remaining dungeon subevents if you want to; I'm just including them for the sake of raising every character's affection as high and as evenly as possible.

Sewer dungeon subevents:

South x27 y22 (Pearl/Emily): Emily is your master? (+Pearl)
South x24 y22 (Pearl/Jordan/Cici): Pearl's right! (+Pearl +Jordan)

West x23 y9 (Summer/Jordan/Cici): This dungeon sucks. (+Summer +Cici)
West x3 y21 (Kairi/Jordan): Sewers suck. (+Kairi)

North x6 y2 (Pearl/Summer): The AGI stat... (+Pearl)
North x27 y12 (Summer/Emily/Pearl): We're family now! (+Summer +Emily)

East x21 y11 (Cici/Emily/Jordan): No choices in this subevent (+Jordan +Cici +Emily)
East x11 y15 (Summer/Cici/Kairi): She should. (+Kairi)
East x16 y27 (Cici/Emily/Pearl): This is a sewer. (+Pearl +Emily)

Sewer story event 1 / Pearl final arc scene (South x19 y27):
(Kairi's final arc scene would also be here if we'd done her previous scenes, but it's mutually exclusive with Pearl's. We can still pick up a couple of affection points with her anyway, though.)
I'll read it aloud. (+Kairi)
Kairi's more OCD. (+Kairi)
Any day, any time. (Unlocks Pearl arc scene)
How're you doing? (+Pearl)
I was scared. (+Pearl)

Final story event (x21 y27):
This leads to the final area of the game and the ending sequence. There are no dungeons or fights beyond this point (although you can still go back before choosing an ending if you want). If you've followed this guide and done all the subevents, your affection levels should now be Pearl 86, Jordan 85, Kairi 85, Summer 85, Cici 91, Emily 94.

Note that choosing "Decide After Hearing Emily Out" is required for Emily's ending, as it leads into her final arc scene. It even gives you some extra affection points with Emily, although they aren't useful for anything at this point until you get her again in New Game+.

When you're ready for the ending:
Decide After Hearing Emily Out (+Emily)
Begone, death's shackles! (+Emily)

Now, just choose "I can't pick one..." to see the harem ending, or any of the other options to see the other endings. Congratulations on completing Undead Darlings!