Rui was a quasi-calm, aloof and somewhat polite individual, rarely raising his voice, and talking with everyone, demon or Demon Slayer, with a reasonable tone and vocabulary, though his low and guttural way of speaking seems to imply repressed anger and annoyance. Every time he spoke with Tanjiro Kamado during their initial duel he remained somewhat civil in speech. He's physically thirteen, though mentally thirty and he's 4'5 or 135 centimeters.