Yui is good cause she is the poster girl. Easy to learn, hard hitter, powerful special attack and an invincible dragon punch (highest priority, more the Eri's).
But all that goes away against a human player, she is pretty useless against Eri, who totally destroys her (human or AI).

Luna is boss, she is special attack dependant but thats fine cause just switching between stances builds her bar like crazy, so just keep poking from a distance and lure em in for the special. Don't let someone like Eri get close though, she will destroy you.