This article contains plot details about the Kunio-kun series. There are spoilers here, just so you know.
Yoshihiro has transferred to Nekketsu High School during Kunio's second year and suggested that they take their class trip to Osaka. When they arrive, Yoshihiro suggests they all go to Umeda. Everyone but Kunio agrees, and they all head off together.
Later, Hiroshi runs into trouble as he's attacked by a group of thugs, and Yoshihiro, who was supposed to protect him, does nothing, instead suggesting they wait for Kunio to arrive. Kunio does and saves Hiroshi.
Later, Yoshihiro informs Kunio that Hiroshi is being attacked again, and this time he's beaten so badly that he's admitted to the hospital. His friend Miho explains that she's forming an alliance to fight against the Osaka Union, and asks Kunio and his classmates to join.
Yoshihiro offers his help, but is later captured by the Osaka Union. However, this turns out to be a ruse: It turns out that Yoshihiro was an inside man for the Union all along. Kunio continues his pursuit of the Union, but eventually finds their leader, Ogura, being killed by Masaki, who, in turn, is killed by Yoshihiro.
Yoshihiro taunts Kunio and challenges him to a fight at the Osaka Harbor. Once there, Yoshihiro reveals his ruthless and murderous nature as he joined the Osaka Union and had their president killed to further his own plans for taking over Japan, and reveals that he transferred to Nekketsu High school for the sole purpose of defeating Kunio. Kunio is unimpressed and defeats Yoshihiro.
Once defeated, Yoshihiro laments that if he had Kunio's power he could rule the world, but Kunio tells him that's not what power is for.
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