Kobushimaru does not play a big part in the story and appears very briefly.
Byakuren no Fangu
When he becomes the enemy of his son, Ranmaru by grasping his neck and shaking him up, they start to have fights at the arena every night. The next morning, they act like nothing has happened. During one of the nightly fights, Kobushimaru is killed by his son.
See more pictures in Kobushimaru/Gallery.