Rui was a boy of short stature and a small physical build. His skin was milky white, with red dots appearing at several spots on his face, located above his eyes where his eyebrows are supposed to be, below his left eyelid, ending above his nose, and finally a vertical one running the length of his right cheek. 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.