Belias and Mateus go to Ashe since (SPOILER)they were Raithwall's servants.

Zalera and Cúchulainn go to Balthier. One is a humongous Grim Reaper clutching a blindfolded woman, and the other is a grinning monster of excess and decay. Awfully fitting companions of a pirate, we thought.

Adrammelech and Hashmal belong with Fran since they're both so overtly animalistic. A ram and a lion.

I'm still slightly conflicted about Famfrit and Shemhazai. I'd like to divide all the Espers evenly (two for each character, even if I don't use them), but Famf's huge cannon is reminiscent of a pirate ship, and Shem is a horse-lady. So they really belong with Balthier and Fran, respectively.