Rui is very innocent and naive, his thinking easily influenced by those around him. He tends to keep a very straight face, and isn't very good at showing his emotions, he could be very happy while keeping a sullen demeanor of a look. He also doesn't talk very much and is very soft spoken, because of this and the fact he's not good at showing his emotions he can come off as very unfriendly and cold-hearted which are both a big misunderstanding of his character. Rui is very kind, and even though shy and distant at first around new people, once he warms up to someone or sees someone in need, he will do whatever it takes to help that person out, even if it meant pushing past his already low amount of energy. People often call Rui very mother-like because of his ways of helping and caring for others in his own way. Speaking back on his low amount of energy, Rui has a very weak body. His body is rather stiff, he has sensitive skin, low blood pressure, weak towards both heat and cold, and noise-sensitivity just to list a few. With being an idol, through dancing and singing he has been able to improve his stamina bit by bit.  

Rui was born into a very prestige family, with his mother being a pianist and his father a conductor. Following his mothers talents, Rui quickly became a musical genius with his main talent for piano. He has an older brother named Rei, who never got the music talent that Rui had and so he became very jealous over his younger sibling.