That's funny, Zevox. I'd probably consider Mitsuru the least mature member of the cast. Her facade is strong and level-headed and in control, but as the game progresses you see that she ultimately doesn't understand how to deal with others on a personal level. She has never had the chance to truly grow up emotionally and it shows. Mitsuru collapses completely whenever the rules and systems she has set up for herself breaks down and immediately turns to others to tell her what to feel.

And Akihiko is just a dork without social skills and a huge competitive streakt which makes him awesome. He is genuinely fun for that reason.