Mikado and Masaomi had been friends since grade school. After Masaomi moved away to Ikebukuro, the two would keep in touch via email. Years later, as the two enter high school, Mikado convinces his parents to allow him to attend school in Ikebukuro. Upon arriving in town, Masaomi takes on the role of tour guide, showing Mikado around while teaching him about all the oddities in town. Two of these oddities include the Headless Rider, and Shizuo Heiwajima.
In the Yellow Scarves arc, Masaomi is trying to ensure that that Mikado won't be dragged into his gang's dispute with the Dollars. This is until Masaomi figures out that Mikado is the leader of the Dollars. Due to a combination of events, Kida is overthrown from his role as leader. The ensuing fight between Kida and the rebel Yellow Scarves is interrupted by Mikado, Celty, and Anri, who all arrive to save him.
In the following few arcs of the novels, Masaomi has pulled back from Mikado, blaming himself Mikado becoming involved in the gang activity. While Masaomi is refusing to meet with Mikado or Anri, Mikado pours himself into the maintenance of the Dollars, reassuring himself that he is carving out a place where him, Masaomi, and Anri can belong together. Seeing this, Masaomi can't bring himself to intervene, watching on as Chikage makes an effort to talk sense into Mikado. Masaomi spends the rest of the novels working in the background to try and save Mikado from himself.
In light novel 8 after Mikado lights Kisuke Adabashi on fire, a stunned Masaomi begins yelling at him due to how unnecessarily cruel Mikado had been. Mikado, paying no heed to his friend's words, just brushes him off before walking away.