TL;DR - while you'll be repulsed by the Tumblr character art, Koa is a decent, if simple 3D platformer with a minor emphasis on speed, but I'll stress that it's not a collectathon. I logged about 6 hours completing everything in it. If you're a fan of 3D platformers, I'd say pick it up on sale.

Koa is a simple 3D platformer with a limited moveset. You can walk, run, jump, ground pound, pick up and throw objects, and roll by pressing the run button upon landing from a jump. The roll grants a big speed boost, encouraging a roll-hop form of traversal which can be pretty fun. However, I'm not sure why the developers even had a run button. Most gaps you need to jump can't be cleared if you're just walking, and there were also times I inadvertently rolled out of a jump when I meant to just resume running.

The game has minimal collectibles, with 1 to 3 unique collectibles in each level, all used to unlock character customizations, boat upgrades, and repairs for the home island. However, all of these unlocks must still be purchased with seashells, the other collectible you'll find throughout the game. Although the tutorial and world map initially seems to imply they're finite, they aren't. I had substantially more shells than needed by the end of the game, so you should collect them casually instead of grabbing every one you see. Finally, there are map pieces, which you earn at the end of each world, and small map pieces, earned from completing races against the other kids. These map pieces, when turned in to Saimi at the town's lighthouse, will clear away some clouds in the world map, permitting you to sail to new islands. Boat upgrades also sort of work to grant you access to more regions of the world map as well as to diving levels.

The world map itself I didn't see a whole lot of point to. Nothing is truly hidden or difficult to reach (there's even a map in the pause menu that shows everything from a top-down view), so they almost might as well have had a standard level select. Yes, you get boat upgrades that enable you to jump over rocks or sail through whirlpools that normally block you, but usually all it does is save you from taking a more roundabout route. There are barrels to crash into for a few shells, and whales to bump into for the same reason, but basically everything you do with the boat is done just for achievements.

The islands themselves host fairly simple levels, with only the occasional gimmick to break up the routine. While the level design is broadly acceptable, I would also say it's not always built around the game's emphasis on speed. The game grants bronze, silver, and gold medals for completing a level within certain time thresholds. This requires you to utilize the roll-hop technique to speed around, but many of the levels have sections that don't seem built around this. Platforms are placed haphazardly without accounting for the distance covered by roll-hops, and there often aren't any alternate routes through the level for those more confident in their abilities. I'd also say the target time for a gold record is inconsistently demanding - for many levels, I cut the gold record time in half, but some autoscroll and boss levels I barely scraped by. I don't believe the developers were allowing extra time for the player to grab all the collectibles in the same run, because you largely can't do that. My playtime nearly doubled as a result, since a second run through a level was basically mandatory for gold records. There's a building on the home island where you can view all of your time records, but it unfortunately doesn't let you jump to the levels to improve your time. But really, you should probably skip getting the gold medals, as there's no reward for the time trials besides achievements, and the leaderboards are full of fake records.

The developers did try to vary levels a little, as there are some 2D sections, some autoscroll levels where you're chased or there's rising lava or where you have to dash, and there's some swimming and diving levels, but I think the game lacks the mechanical depth needed to keep things interesting. And I don't think that's because the developers were keeping things simple for a younger audience, because there's enough in the game that seems a bit too challenging for their age.

The character designs are... ugly. Varied, but ugly. I also noticed a real inconsistency in the art style between the 3D models, the character portraits, the trailer, and the title screen. Maybe the developers recycled art from their other games? It was distracting, anyway. The game tries to build up the idea of a close-knit community where everyone knows each other, but it takes place across a bunch of islands completely empty of people outside of boss fights or one recurring character. Only the one island really has any people, and you only come back to it for upgrades and level access. More people will appear on the island after you pay to have stolen items returned, but they're all nameless NPCs. There are no story events when you return home after collecting a map piece, and no one really changes what they have to say, so the developers haven't created the sense of community they intended. Not that story is ever that important in a platformer, but you know. The music is inoffensive.

But so is the game itself. There's nothing exactly daring or stand-out about it besides the hideous character art. It's simple fun, but it's also got nothing that makes me want to come back to it again. If you like 3D platformers and find it on sale, it's a decent way to pass the time. Just don't waste your time on the achievements, even though they are quite doable.