Shishunki Danshi Junjoukei

A violent older brother named Maki suffers amnesia after an accident on graduation day and regresses to the personality of a 6-year-old girl, despite retaining his adult body. His younger sister, Mayuri, initially finds the change unsettling, especially as their family reacts differently to Maki. Over time, Maki's childlike behavior begins to affect Mayuri, and after defending Maki from gang members seeking revenge, he starts to experience confusing emotions. The story explores the evolving relationship between the two siblings as their bond shifts in unexpected ways.