In your Steam account, the game progress of each playable characters is independent, and of course contains attribute points. (Except for some tools: bow, boomerang, and shovel. As long as any playable character get the tools, and then return to the world map for the system to automatically archive, all playable characters in your Steam account will receive these tools permanently.)

Any playable character with Lv 1 has health point (HP) =100, and all of the four main attributes (Strength, Magic, Defense, Agility) are 1.

The maximum level of playable character is Lv 99.

Each time you upgrade characters' level, you will get assignable attribute points.:
Two assignable attribute points per level gained until upgrading at Lv 20.
After that, one assignable attribute points per level gained until upgrading at Lv 78.

You can allot the points on four main attributes: Strength, Magic, Defense, and Agility.

You can invest up to 24 assignable points per attribute. So, if the effects of weapons and animals orbs are not included in the calculation, the maximum value of each attribute is 25 (from the initial 1 point plus the assigned 24 points).

Playable characters can increase or decrease their attribute value by carrying weapons and animal orbs, so the actual maximum value of a single attribute can exceed 25.

Although this means that all characters can get enough points to fill all the attribute slots at Lv 78, there is still a concern that improper allocation of attribute points in the early game may lead to a unpleasant game experience. On the other hand, the higher level upgrade, the more experience points are required. So it's still crucial that the attribute points are carefully allocated in the early game.

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Affects damage from melee attacks.

Affects include the maximum MP, MP recovery speed, and the number of magic skills that can be used.
For every 5 points of player character's Magic, you can gain one more magic skill and increase the maximum distance of Splash Attack.
The type of magic skills is the biggest difference among all playable characters.
For more complicated information, it is recommended to refer to: https://castlecrashers.fandom.com/wiki/Magic

Affects maximum health points (HP) and damage resistance.

The playable character's HP = 69 + character level × 3 + Defense × 28.
Therefore, the initial playable character with Lv 1 has HP = 69 + 1 × 3 + 1 × 28 = 100;
For a playable character with level 99 and already fill all the attribute slots by assignable points, HP = 69 + 99 × 3 + 25 × 28 = 1066.

For each one point of Defense, the character gains 1% damage resistance.

Affects include the playable character's movement speed, archery damage, arrow flight distance, and archery shooting speed.

Here is a concept applies to any playable character:
Benefiting from the ability to obtain high mobility, high damage output and long-range attack capabilities at the same time, I strongly recommend investing all assignable attribute points on Agility in the early game. Can be full of Agility as soon as the character Lv 13 (the arrow does not land, and the rate of fire is super fast!).

Strictly speaking, the difference among all playable characters in Castle Crashers is almost only in the type of magic skills, and other aspects are almost the same. For example, with the same Agility, there is no difference in the movement speed and archery-related abilities of all playable characters.

Whenever the level is increased to a multiple of 10, the playable character's melee and magical damage in combat will increase slightly. (This additional damage bonus is not displayed on the attributes value panel)

Playable character's level also affects some magic skills and combos of melee attacks that character can use.

The MP value of the character is a hidden value, and I'm not sure whether it will be passively increased by the character's level upgrade.

"XP" in the game is mean "Experience Point", which can be obtained by:
Each time you hit an enemy whose HP has not dropped to zero, you gain 1 XP.

In other words, "attack corpses" will not receive additional XP. In addition, the amount of XP obtained has nothing to do with the type of enemies, the damage that is given, and whether it has given the enemy a last hit.

If you want to know more detailed settings for Castle Crashers,
I strongly recommend checking out the "Castle Crashers Wiki", which has the most complete information:
https://castlecrashers.fandom.com/wiki