Let's talk about one character at a time.
I think the character is well-balanced and there is no reason to make changes to it.
Even though it is fast, it remains very fragile and its greatest advantage will always be small and fast.
Her skill helps but it's dangerous and you can not just walk away or get killed easily.
This character has a lot of advantage.
It has a good speed and HP and still has its automatic regeneration.
It is very easy to recover your HP again.
There is no need to add more HP.
If you have automatic re-generating power there is no need to increase your HP.
HP will rise naturally as you evolve your char and the power of regeneration will continue forever and Ai will become more and more powerful.
If you use items and HP Recovery + Automatic Regeneration this will fill your HP quickly.
If Ai's HP increases this will start to become disproportionate to his regeneration ability.
The ability to regenerate is not a skill like Stun (Burton) Jump (Carl) or uplo bearing (Marie) is just a gift and only Ai has it.
It is the character that counts on the best abilities.
I do not see weaknesses in an Ai.
He could have as fast as Marie.
For being small and have a great jump and so it takes speed to be able to use your skill properly.
Carl's initial speed is too slow.
HP is stable and good.
If we increase the HP of all the characters what will be the advantage of Burton?
There's no need to mess with Carl's HP, Ai and Marie.
I would put more MP and Speed for Carl.
I do not understand the need for BURTON to be so slow.
He's a character to be on the front line and needs some speed.
If a Burton is too slow he will not have agility to use his skill (Spear).
Even rolling the Burton is slow.
Is it large and easy to hit then because they slowed down?
In this way Burton will no longer be a front-line character and will start to be a more defensive character.
How can a Burton stand in front of Marie and Carl who have a lot more skill?
The only advantage I see now is when you use the Spear skill and take a shot and the Spear skill considerably reduces the damage.
This is good because you can face the enemy up close and use the skill to make the enemy dizzy and still reduce the damage it took while doing the skill.
But with low speed it is difficult to reach a Marie when she uses the double bearing or Carl when it jumps over you.
It would put back the 5% speed and removed from Burton and increase its MP so that it can use more skill Recovery because its HP is big and difficult to fully regenerate life in combat.