[@RumikoOhara] Since Trey's more the frontline fighter, with his speed and strength letting him get into combat quickly and hit the enemy hard, I'd suggest giving him Fiore as a partner, since he's more likely to get injured either from enemy action or from overexerting himself. This would mean Trixie would be partnered with Kiki, which would work out welol. Trixie's more gung-ho and could protect Kiki with her Multitool, while at the same time providing her with plenty of robot minions to control.

Personality-wise theis works out too. Trixie's more bombastic outlook on life would help ground Kiki, giving her a focus to help keep her attention in tense moments. Trey's calmer, more disciplined personality would clash more with Trixie's, and would complement Fiore's kind and optimistic one better.