Her personality is due to her parents divorcing. She was closer to her dad, and her mom doesn't really understand her, always pushing the tomboyish Ruki to try and be a model. This left Ruki feeling isolated, and forced her to be defensive and rebellious. As a self defense mechanism mentally, she believes she "doesn't need anyone". Which is believed to account for the way she treats Renamon at first. Deep down, Ruki is loving and caring: wishing to protect her friends as well as the city. Even though she puts on a "defensive act", being tomboyish is a genuine part of her personality.