Just gonna wiki because there is better information.
Rsident of Ikebukuro and Mikado Ryƫgamine's best friend from childhood. He is the one who encourages Mikado to come to Ikebukuro. He is enthusiastic about picking-up girls, but seems to be a failure 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 the Yellow Scarves, which he led at the time. After, Anri Sonohara is injured during the Slasher incidents and the rising presence of the Dollars, Kida is forced to become the leader of the Yellow Scarves once more.
Kida despises Izaya Orihara 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 has a very friendly, enthusiastic, and outgoing personality, and is almost always seen fooling around, smiling, making bad jokes, teasing friends, and hitting on girls. He is shown to care deeply for his friends, having to act as a sort of big brother for Mikado as he adjusts to life in the big city and going to great lengths to protect those who are close to him. When Mikado first sees Masaomi in Ikebukuro, after years of separation, Masaomi's bubbly personality and bad jokes are what convince him that the man greeting him is indeed Masaomi Kida. As the series progresses, it is revealed that Masaomi's enthusiastic, happy-go-lucky demeanor is actually a mask of his true, much less secure self. He appears to agonize over major choices much more than any other character in the series, although he falls short of being paralyzed by his fears, still being able to make important decisions when necessary. He also demonstrates a feeling of responsibility for those close to him, taking attacks on his friends very personally. Despite appearing carefree, his experience as leader of the Yellow Scarves makes him painfully aware of the realities of life, and his greatest fear appears to be allowing himself or his friends to "cross over to the dark side," preferring that they stay completely away from the color gangs even to the point of being ignorant of them.
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, The Blue Squares, 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 the Blue Squares allegedly played dirty and only picked fights that they could win, devastating the Yellow Scarves.
It was during the Yellow Scarves/Blue Squares 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 Squares 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 the Blue Squares 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 the Blue Squares 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 the 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 to be better that Mikado would 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.