Shun Uruma Sr. is an elderly man who is a former member of the Kitayama corps, a special WW2 unit which specialised in assassination, he's the father of an unnamed man and grandfather to Shun Uruma and Kakeru Uruma.

Spoiler

After the tragic murder of Shun Uruma's parents and the severe injury of Kakeru, Uruma Sr. finds Shun Uruma attempting to exact revenge on his bullies that caused the tragedy, and decides to train him in military combat.