Second son of Yamada brothers, also known as MC.B.B. He's one of the Ikebukuro Division's Boss.
Jirou idolizes hig big brother Ichirou so much that he became a delinquent himself. He's a strong fighter and will crush anyone who makes fun of his big brother. Thanks to Ichirou, Jirou is well-versed with anime and light novel.
He's constantly fighting with Saburou, who also idolizes their big brother Ichirou.