Well I didn't really think about it that much,but:
Belias- Ashe,again Dynast King descendant so I thought it would be better to be given to Ashe.
Mateus- Ashe,Idk she was my fav character back then.
Adrammelech-Penelo,because she was the character who pretty much defeated him,when I fought so I was like,'why not?'
Zalera- Hmm..I gave him to Balthier.Cuz since he was the leading man,I thought he would be the skeleton dude and the damsel be a idk..a damsel that Balthier had to rescue,guess u could call it wishful thinkin.
Zeromus- I think I gave this one to Vaan.Dont ask me y.
Shemhazai- I gave this one to Fran.One because she didn't have one yet,and two..uhh..I think she killed him,too.
Hashmal- I think I gave this one to Fran as well.Idk..I guess I was bored and didn't care.
Famfrit-Balthier,for reasons I don't know.
Exodus-I either gave this one to Fran or Penelo.I can't remember.
Chuchulainn- This one I think I gave to Basch.Again,IDK y..
Chaos- This one I gave to Vaan,for reasons again IDk
Ultima and Zodiark I haven't obtained yet,but I am working on it.
*U know,like some of u have said,the Espers in this game..they were actually a shame.Because they didn't help as much as the Aeons and Gf's in FFX and FFVIII.