Izuku Midoriya, the protagonist of the popular manga and anime series “My Hero Academia,” has been calling his childhood friend and rival, Katsuki Bakugo, by the nickname “Kacchan” since they were young. The reason behind this nickname has been revealed through various moments in the series.

Bakugo’s Explosive Quirk:
Bakugo has a Quirk that allows him to create explosions from the sweat on his palms. As a child, he would often use his Quirk to intimidate and bully other kids, including Izuku. However, despite his abrasive and aggressive personality, Izuku has always seen Bakugo as someone with great potential.
Childhood Friendship:
Izuku and Bakugo have been childhood friends since they were very young, and Izuku has always looked up to Bakugo’s strength and confidence. However, their relationship became strained when Bakugo began to bully Izuku for being Quirkless. Despite this, Izuku continued to admire Bakugo’s determination and drive, and he refused to let their friendship end.
Nickname Origin:
The nickname “Kacchan” was given to Bakugo by Izuku when they were kids. The origin of the nickname is revealed in the manga and anime during a flashback to their childhood. In the flashback, Bakugo is shown to have a very explosive personality, and he would often get angry and yell at other kids. Izuku found this behavior amusing and started calling him “Kacchan,” a shortened version of “Katsuki-chan,” a respectful way of addressing someone in Japanese. Bakugo hated the nickname at first, but it eventually stuck, and he now seems to accept it as a part of his identity.
Symbol of Friendship:
Despite Bakugo’s rough exterior and tendency to bully others, he and Izuku have a deep bond that has grown stronger over time. Izuku continues to call Bakugo “Kacchan” as a symbol of their childhood friendship and the respect he has for Bakugo’s strength and determination. The nickname has also become a sign of their complicated relationship, as it reminds them of their shared past and the challenges they have faced together.
In conclusion, Izuku Midoriya calls Katsuki Bakugo “Kacchan” as a nickname given to him during their childhood. The nickname originated from Bakugo’s explosive personality and has become a symbol of their friendship and shared history. Despite their rocky relationship, the nickname is a reminder of the bond that they share and the respect Izuku has for Bakugo’s strength and determination.