All the characters levels are worked out differently...

Balthier and Fran are Vaans level + 1.
Basch = Vaans level + 2.
Ashe = Average of the two highest levels + 1.
Penelo = Same as Vaan first time you fight with her, then the next time she joins its the average of the two highest levels.

I wont bother with the guest and support characters.

Is that what you were talking about, their level when they join? Otherwise ignore this lol.