IIRC, with ToS, all the characters level up at roughly the same rate, even if they don't get used in battle. (Personally, I like controlling Sheena, since she has fast attacks and is more of a challenge than Lloyd. Presea I almost never control, since I don't like how she handles. I'm very OCD about switching characters into the battle party regularly, even if there's no real advantage beyond getting them to use their techs enough to unlock the next set.)