When leveling up 2* characters ALWAYS use copies of the character you are leveling up. This gives you three bonuses:
- first is some bonus experience for using the same color (for example Slice)
- second is a chance to level up the character's skill - since the characters sacrificed have the same skill as the upgraded character
- third is unlocking the additional equipment slots - because you are using copies of the same character

2* characters are rather weak, but with level in the 8-11 range, skill at level 5, and 4 equipments boosting their stats they can be quite useful at the earlier mission, until you manage to get some 3* or better characters.