Leader of the yakuza group Kuryugumi at the age of 14 due to his father's assasination, Ryuuji looks much older than he is (due to his height and almost constant lack of facial expression). Even though he didn't cry or show any signs of grief after his father's death, when Tsukasa claims she can identify the killer (who also happened to be her own parents' killer), he jumps on the opportunity. As it turns out, he already knows Tsukasa's secret (that she's really a girl), much to her surprise. He has also had quite an interest in her for a long time, without her knowledge, being envious of her easy smile and attractive aura.
After seeing Tsukasa in girl's clothes for the first time (i.e. finally seeing her actually as a girl), Ryuuji decides to come up with a "debt" based on her brothers' food consumption in order to make her stay on as his bodyguard (to pay it off), which he constantly increases by charging for all the food Tsukasa eats, anything she breaks, any bill that relates to her at all, etc. - all of this simply to keep her by his side for as long as possible. In addition, he takes to flirting with her quite openly, even though she never seems to catch onto his motives (being rather dense in that area).