Ann's Persona gives her mastery over fire. Her flames produce some of the hardest-hitting attacks in the entire game. Ann being a Fire-type Gym Leader is a no-brainer. As a character, she feels like a bit like a mix of Flannery and Elesa from Pokemon.
Ann seems pretty calm and quiet at first, but once you get to know her, you learn that she has a more lively and outgoing personality. She isn't as over the top as Ryuji, but Ann is quick to stand up and speak her mind in matters concerning her friends.
With all of the above traits, Ann would have a team like this: