Osamu is Tatsu's childhood best friend and Shuuji's best friend too. He is a member of Izumokai Youth Division and later on he became the leader after Kubota left. Before he was introduced officially, he was actually in Dice 2 asking Shuuji if he was alright after being disputed by Toujougumi.
He was later introduced officially in Dice 25.5 where he's with Shuuji and Tatsu. They were telling/scolding to Shuuji to give up playing arcade since they are in the Toujougumi's territory. They asked him to go back to the office before they left Shuuji.
Off-shore oil tanker voyageEditEver since Shuuji's death, Osamu was acting differently. He was always having a nightmare about Shuuji's death. He was later ordered by Sanada to kidnap Kubota's precious cat to gather information about W.A.
After 3 days, Tokito was kidnapped after he leaves Kou's shop. The Izumokai Youth Division hid Tokito in their oil tanker ship. He and his men were violently injuring Tokito forcing him to talk about W.A. and Tokito simply that he doesn't know anything about W.A. Osamu even called him a monster.
Later on, Kubota attacked the ship all by himself and killed Osamu's men one by one. Osamu tried to contact the main office, but the connection was cut by Kubota. Tatsu argued with Osamu that for Kubota they are just stray dogs and if they we're cut down, everything they did will be useless, but Osamu simply if they didn't move in their world, they won't able to face Shuuji after life. Tatsu got mad even more and shouted that it is not what Shuuji wanted and he and Shuuji weren't cunning like Osamu. Osamu calmy responded that he only chose the path of survival and it's no good to be cunning.After that, more deaths were happening in the ship by Kubota. One of the members, Matsushita was scared and Osamu ordered him to calm down. They later heard a gunshot from below where Tatsu was guarding their hostage, Tokito. Osamu immediately run to it when he saw Tatsu on the ground and completely injured. He was begging Tatsu not die but Tatsu released his hold meaning his life ended. He later realizes that his team was almost sinking and asked himself if where did they go wrong? In the end, Osamu along with Matsushita who was the only two persons alive in the ship encountered Kubota asking him to take responsibilities for what he did to his men. Kubota threw his gun on the water but grabbed the gun of Osamu and killed Matsushita. Kubota was about to killed Osamu, but wasn't able when the helicopter of Sanada came.
Osamu was seen to be alive in ActiBook Dice 43, the revision chapters by the author when she transferred the series to Ichijinsha. Behind Osamu, there are members of Izumokai and Tatsu's back can be seen.