Kirisame is mostly seen in flashbacks, being one of the children in Jaula Blanca Institute. During the attack by the Reggies on the Institute, Dr. Kazame orders Kirisame to take Koku to a different shelter in the woods where he promises them they will both be safe. After convincing Koku to leave behind his friends, specifically Yuna, Kirisame and Koku run into the woods. Just as they reach the tree shelter, they are attacked by the soldiers. Kirisame tells Koku to go into the shelter and lock the door, telling him he will see him soon. Kirisame's arm turns into blue steel and he runs to attack the soldiers with some of the other children that arrived on scene.When Koku leaves the tree shelter after the fight, Kirisame seems to have died from wounds, along with the other children. Koku goes back to the Institute, and after receiving instructions on what to do, cuts off Kirisame's arm so he can have that ability.
Kirisame's arm of blue steel sword is then used by Koku all throughout the show. During Koku's fight with Lacia/Minatsuki, Koku tells Minatsuki that his friends, including Kirisame, are living on within him.
In the post credits scene of the last episode, Kirisame is revealed to be alive. He admonishes an unknown person, telling him that the failure of taking down Koku was because they left it to a Reggie. He then states that he has been hoping to see Koku, to get his arm back, and that he will keep his promise to Koku of seeing him soon.