You got Charon before fighting the Star Magician? Since you have to get all the Djinn to get Charon, I usually clear the dungeons when I pick up the Djinn.

My party is well rounded. One Adept of each type. Mia is better than Piers, IMHO, because she learns Wish, Wish Well, and Pure Wish with Water Djinn, while Piers has to mix and match to get those. I prefer sticking each Adept with their main Djinn types.

Isaac (awesome character, and a better guy than Felix, IMHO)
Jenna (another awesome character, I support the Jenna/Isaac pairing, and bad@$$ Psynergy. Cool Aura, Dragon Fume, Searing Beam)
Sheba (I just prefer Sheba to Ivan. Not much difference between the two)
Mia (best support character in the game, with decent attack Psynergy)