From the Heian era, you and your older brother, Muzan, were both born with the same disease. The disease caused you wouldn't live more than 20, and you were clinging to your brother since childhood, while you were 16, and him 19.

The doctor discovered a medication made from flowers - specifically, the Blue Spider Lily - but the doctor died after your brother stabbed him in a fit of rage, because nothing changed.

"Stupid doctor.." Your brother said, still clenching his teeth in anger.