Mikado and Masaomi had been friends ever since grade school. After Masaomi moved away to Ikebukuro they e-mailed each other every day. Then Mikado moves to Ikebukuro because Masaomi asks him to. In Ikebukuro Masaomi shows Mikado around and tells him about the people in the city. In the Yellow Scarves arc Masaomi is trying to make sure that Mikado won't be dragged into the mess until Masaomi figures out Mikado is the leader of the Dollars. When Masaomi gets disbanded as the leader of the Yellow Scarves he is also told about the Yellow Scarves going to hurt Mikado so Masaomi runs into the base in order to save his friend and show that he is not afraid anymore. When Masaomi is beaten severely Mikado holds him up trying to make sure Masaomi won't die. In later light novels Masaomi is trying to turn Mikado back to normal because of the Dollars and Blue Squares forming and Mikado not acting like himself.
In light novel 8 after Mikado lights Adabashi on fire, a stunned Masaomi begins yelling at him due to how unnecessarily cruel Mikado had been. Mikado, paying no heed to his friends words, just brushes him off before walking away.