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.
Masaomi founded the Yellow Scarves when he was in middle school, intending it to be a playful sort of team. As the Yellow Scarves grew, they became one of the notorious 'color gangs' that went around the city, occasionally fighting each other. However, as the gangs grew older, one of their rival gangs, Blue Square, began to get serious, stretching the boundaries of a typical gang of middle schoolers and becoming quite violent. The Yellow Scarves naively expected a fair battle, but Blue Square allegedly played dirty and only picked fights that they could win, devastating the Yellow Scarves. It was during the Yellow Scarves/Blue Square war that Masaomi met Saki Mikajima, a girl who recommended him to an informant, Izaya Orihara to help him fight his wars. Masaomi was, quite reasonably, put off by Izaya's personality and found it difficult to trust him, but Izaya's advice was sound, giving the Yellow Scarves intel on Blue Square hangouts and teaching them how to fight. The success Izaya brought to the Yellow Scarves led Masaomi to unintentionally become dependant on him for help, and Izaya continued to provide information until Blue Square was cornered and about to be finished off.
Thanks to Izaya, Masaomi and Saki saw each other and talked a lot, eventually deciding that they were, in fact, dating. Unfortunately, Saki was kidnapped and tortured by Blue Square in an attempt to lure out Masaomi, and end the Yellow Scarves. Masaomi sought Izaya for help, but Izaya would not answer his phone, and Masaomi was left helpless and unable to save her. Fortunately, Kyohei Kadota and a group of Blue Squares who were disgruntled by the gang's lack of honour betrayed their gang and saved Saki, although she had to be hospitalized for a year.
Masaomi left the Yellow Scarves after the war, and vowed never to go to "the dark side" again. However according to Izaya, the incident made Saki his "god," with her becoming the embodiment of his dark past that he could never escape from.
It was through chats with him that Mikado Ryuugamine decided to come to Ikebukuro, lured in by the sense of excitement and fun that his friend Masaomi described (Masaomi left out the harsher details of his life in the big city, including his gang experience, as he believed it better that Mikado never find out about them). Hearing about how bored he was with his small town life, Masaomi encouraged Mikado to join him and enroll in Raira Academy.
Protecting and teasing Mikado for his naiveness became a hobby for Masaomi, and helped him become a little less depressed about his life. On the first day of school, Mikado followed Anri Sonohara in volunteering to be class rep; Masaomi, and many other guys, were amazed at his apparent boldness (Anri is considered by many to be quite attractive, and the implication was that Mikado was going after her. Mikado however insists that he had other reasons). They all became friends after that, and started the trio of protagonists of the series, which Masaomi playfully describes as a love triangle.
It wasn't until much later in the series, when the Slasher incidents began, that Masaomi had another run in with the underworld. When Anri was attacked by the Slasher (it was actually one of Saika's "children" who attacked her; Anri is the original Saika and she had nothing to do with the Slasher incidents, although all of this is unknown to Masaomi), Masaomi was enraged at the attack on his friend and felt the need to use the Yellow Scarves power to hunt him down. The Yellow Scarves were quick to welcome back their old leader, although Masaomi found the gang to have changed drastically.
As rumours went around that the Slasher was part of the mysterious "Dollars" gang, Yellow Scarves started hunting down Dollars members, although this proved difficult due to their characteristic "invisibility." Tensions rose sharply when the feared Black Rider was seen helping someone with a Japanese sword escape a group of Yellow Scarves; the Black Rider was a known member of the Dollars, and this was seen as solid proof that they were in league with the Slasher. After this, the Yellow Scarves, still strong after years of laying low and gaining strength off of the resentment the city felt towards the Slasher, fully mobilized, hunting down everyone who was suspected of being with the Dollars and beating them up.
Of course it was unknown to Masaomi that the Dollars were actually 'led' by Mikado, and that Anri was now the Slasher (and fully in control of herself). He still disagreed with the unneccesary violence against innocents, but his gang was out of control, and ignored his orders to stop, continuing to assault the Dollars while the Dollars did nothing in response. Masaomi grew fed up with the clear insubordination of his gang, and began sensing their mutinous intentions; his suspicions were confirmed one day as he walked into the Yellow Scarves base to find Horada in his 'throne,' blatantly taking control of the gang. It turned out that as the Yellow Scarves started letting anyone join them (this was to allow those who opposed the Slasher to fight him/her), Blue Square had actually infiltrated the gang. The fact that Anri was using Saika's children within the Yellow Scarves to try and stop attacks against Dollars members helped create a sort of chaos within the gang where members started fighting each other; Blue Square took this as an opportunity to 'remove' those still loyal to Masaomi and place those still loyal to Blue Square within the gang. Horada put out an execution order for Masaomi to be carried out by his own gang; fortunately Celty Sturluson (the Black Rider) found out, and brought Mikado and Anri with her to save Masaomi. Masaomi proved able to hold his own in battle, taking on several Yellow Scarves at once, and was able to survive long enough for his friends to come to his aid. It also turned out that Kyohei Kadota and his group, now with the Dollars, had also infiltrated the Yellow Scarves, and joined Masaomi's side in the battle. It was after this battle that all three friends (Mikado, Masaomi, and Anri) learned of each other's parts in the whole war and came to terms with it.
Although Masaomi survived the battle, the execution order against him still held, and the Yellow Scarves were still strong, so it became impossible for him to freely show himself around the city. He had been avoiding Saki because of the guilt he felt for her injuries, but his scrape with death led him to visit her to try and repent; Saki surprisingly didn't mind his absence OR his habit of flirting with other girls, as she believed that he would always find he loved her more. She also revealed that she allowed herself to be kidnapped by Blue Square on Izaya's orders, and as usual she followed his orders unquestioningly. It turned out at some point, she actually realized that Izaya was simply playing destructive games with people for his own amusement (Masaomi becoming dependent on him, Saki being kidnapped, and him suddenly ignoring Masaomi's calls for help are an example. The Slasher/Dollars/Yellow Scarves war is another, although he had other reasons for spurring that one). Towards the end of the war, she overcame her idolization of Izaya and sold him out to Simon Brezhnev. Although surprised, Masaomi revealed that he also knew that Saki was faking her inability to walk. Eventually they both forgave each other for their actions, and Masaomi overcame the paralyzing guilt that forced him to stay away from Saki, and the two disappeared from Ikebukuro together.
After the main story, a mysterious user named "Bakyura" became a member of Mikado's private chatroom (at this point the chat included Mikado, Anri, Celty, and Izaya, although not all the users were aware of the identities of some of the others). The way he spoke, his general demeanor, and the timing with which he joined led Mikado to suspect that this user was actually Masaomi; although Bakyura ignored Mikado's inquiring PMs, he repeated jokes, Masaomi liked to make with Mikado (at Kanra/Izaya's expense). Although Masaomi and Saki's whereabouts are unknown, Mikado states confidence that Bakyura is Masaomi's way of staying in contact, and that the friends will meet again in the future.