Rui, the Lower Rank Five demon from Demon Slayer, is physically and emotionally portrayed as a 12-year-old boy, but his actual age is much older due to his long life as a demon. In human years, before his transformation, he truly was around 12. After becoming a demon, he lived for an unspecified number of years—likely decades—though his appearance never aged because demons in the series remain frozen at the age they were turned.
This gap between his human age and his demon lifespan is part of what makes his character unsettling. He retains the fragile, almost innocent look of a child, but his mindset, cruelty, and twisted concept of “family” reflect years of experience and detachment from human morality. His fixation on controlling his spider “family” comes from his warped longing for the human connections he lost, which is only intensified by the unnatural longevity of his demon existence.