Futaba is another tricky one because she doesn't participate in battles as a playable character. She's always around once she joins the Phantom Thieves, but she's in more of a support role. She provides intel for the team, instead of joining them in combat.
She's a bit awkward and a little shy, but once she joins up, Futaba becomes the brains of the operation. She relays information and even turns bad situations into good ones before disadvantageous fights can break out.
Futaba's weapon is her high intelligence, and because of that, she has to be a Psychic-type Gym Leader. These are the six Pokemon she'd use: