I also guessed this(Come on, she is only a version less stupid-and more immature of Usagi). By the way, the story is driven by her conflicts(the others are only episodic) and her personality, being a Genki Girl with Love Freak/ The Messiah and mischievous tendencies,hardly is the more emotionally balanced(actually she is the id and the red oni in a Red Oni Blue Oni when compared with all less Minako), she hardly sounds serious(actually, she is the less disciplinated of the group!)In other words, she hardly fits.