Yamane is a member of West Mifune who loves baseball but loses enthusiasm after his senpai teammate intentionally injures him in a practice to take his starter position. He is also upset because the new captain does not recognize he is still injured and assumes he is okay after his cast is removed, but he can not throw with his right arm anymore without pain. He begins to hate baseball, wanting nobody else to enjoy it. He begins to cut classes and grows his hair longer.
He brings delinquents onto the team, chases off the other members, tries to prevent new people from joining and vandalizes the equipment while using the club room as a place to ditch class.. That said, he disapproved of his flunkies actually inflicting violence on the players themselves.
After Goro finds out about the equipment, he and Yamane get in a fight, where his motivations are revealed. Yamane is motivated by how Goro got past his injury and by the captain's apology, and also begins to train with his other arm.
His number is 3 and he plays first base.