I play pocket Mitsuru, I'm speaking from experience. She's very easy to pick up and learn the game with compared to the rest of the cast IMO.
Chie will give you better anime fundamentals, but Mitsuru is overall I think the easiest character to get more bang for your buck in terms of training mode stuff. She's got good easy B&Bs and will teach you to translate random hits into damage very well.
Entry level -> mid level Mitsuru is fun and will teach you a lot.