A resident of Ikebukuro and Mikado Ryūgamine's best friend from childhood. He was the one who encouraged Mikado to come to Ikebukuro. He is enthusiastic about picking-up girls, but he seems to fail at it. He was depressed and lonely before Mikado arrived because his girlfriend was kidnapped and injured during a gang war between the Blue Squares and Yellow Scarves, which he led at the time. After Anri is injured during the Slasher incidents and the rising presence of the Dollars, he is forced to become the leader of the Yellow Scarves once more.
He despises Orihara Izaya for manipulating him when he first formed the Yellow Scarves, but returns to him again for information about the leader of the Dollars and discovers it is Mikado. He leaves Ikebukuro with his ex-girlfriend, Saki Mikajima, during the conflict between the 3 main forces in Ikebukuro. Masaomi has commented on not letting Mikado "go to the dark side", claiming that he has already gone through it.
Birthday: June 19th ("the Day of Romance" in Japan)
Blood Type: A
Hobby: Picking up girls
Favorite subject: English
Least favorite subject: History
Foods he likes: Anything as long as it's a meal with girls
Foods he dislikes: Everything if it's a meal with just guys
Favorite saying: "Love is a leveller."