“You know, I always looked down upon you ‘cuz your quirklessness. I thought you were beneath me. However, I felt that you were higher than me. It was horrible. It was too painful to look at you. I could not accept you as you were. That’s why I didn’t accept you as you were. I tried to be superior by refusing bakudeku. That fight was futile. We got into U.A Everything didn’t go as planned. This past year has helped me understand your strengths as well as my weaknesses. While I don’t think this will change anything between us, I do expect it to make me speak my truth. Izuku… I am sorry for everything.”
Canon Bakudeku
As the series reveals, Izuku and Katsuki know each other since childhood. Their relationship became strained over a series misguided events. Katsuki even forced Izuku to leave because he was afraid of Izuku’s spirit. As they grow older and work together to solve their problems, their friendship begins to heal. Katsuki is even encouraging Izuku to be stronger in recent chapters. Izuku still admires Katsuki despite their differences, particularly his strength as a hero and determination. He says that Katsuki is his true image of victory.
The nicknames they use for each other are a result of their childhood friendship. Katsuki, despite his negative attitude towards Izuku allows Izuku the nickname “Kacchan” without objection. Katsuki’s explosive and brash nature is clearly reflected in the suffix “-chan”, which is cute and feminine. It is clear that Katsuki will make a comment about any issue he has with someone or something, even if it is Izuku. Katsuki seems to believe that the nickname is a small part of their childhood that is unaltered. This is why Katsuki doesn’t make any arguments. Kaminari used “Kacchan” in chapter 163, apparently in a friendly joke manner. Katsuki ignores this completely and goes to bed. Horikoshi, creator of the series refers to Katsuki occasionally as the nickname in his sketches but does not reveal much about its history or origin.
Katsuki gives Izuku the nickname “Deku”, which is essentially an acronym for “useless person” and refers to Izuku’s quirky state. Ironically, Izuku eventually gets this name from Katsuki, his classmate and friend. Ochako suggests it and says that it reminds him of the phrase “you can achieve it!” Katsuki initially feels anger at this and believes it to be Izuku mocking his character. However, she doesn’t make any comment.
Katsuki sees Izuku bakudeku as someone less than him since the day he failed to create a Quirk. Katsuki was encouraged to develop his own Quirk, which was repeatedly praised by his teachers and classmates, while Izuku was reprimanded. Katsuki, a naturally influenced child, noticed a dramatic difference in how they were treated. Izuku was considered a nobody in an environment where Quirks are so important to defining a person’s personality. Katsuki believed he would be the best and continued to see Izuku only as a “pebble” on his journey to becoming a hero. His intense attachment to winning made him feel somewhat superior.
Season 1 Bakudeku
Izuku rushes down to Katsuki during the scene where Katsuki is thrown into the river. Katsuki is offended by this because Deku, the quirkless child who was clearly lower than him, was allegedly looking down at him. Katsuki bullied Izuku over a number years, using Izuku to vent his anger at his desire to be the greatest hero. Katsuki became aggressive after learning that Izuku wanted to enroll in U.A’s Hero Course. He mocked his decision and said that he should “take a swan jump off the school’s roof”. Izuku was in an angry stupor when he said this, but he still worries about Katsuki’s impulsive actions. He stated that Katsuki could have gotten into trouble had Izuku actually jumped from the roof. Katsuki later learned that one of his friends had told Katsuki, while Katsuki was with his friends, that he had gone a little too far, which meant that it wasn’t his normal behavior.
Izuku is on his way back home when he sees an explosion, a crowd, and what appears to be another villainous event. He soon discovers that a boy (Katsuki), had been captured and held captive by the same slime monster from which All Might had saved Izuku. Katsuki tried desperately to free himself from the pain as his limbs and mouth were engorged. Katsuki stared in despair as he felt unbearable pain. Izuku saw the situation and ran towards him immediately, regardless of how dangerous. He quickly thought and threw his backpack at slime monster. One of his books flew out, and Katsuki was able to breathe again after it hit the slime monster’s eye. Izuku continued to claw at the slime monster frantically. Katsuki was furious and asked Izuku why he was there. Izuku replied that his legs were “moving on their alone” and that Katsuki seemed like he “needed be saved”. All Might and other Pro Heroes, who were watching, intervened to save the two and defeat the villain.
Katsuki stopped Izuku as he was walking home from the sludge incident and said that he didn’t need his assistance and would be fine on his own. Izuku was also instructed not to look down upon him. Katsuki turns around and walks off, while Izuku looks on with a happy expression. Katsuki did not harass Izuku like usual. Instead, he merely threatened him. Izuku, however, was happy to acknowledge that Izuku’s bravery saved him.
Izuku also stated that Katsuki has never harassed him physically since the incident.
Katsuki’s misinterpretations of the Quirk’s origin made their relationship even more complicated after Izuku was granted the quirk One For All from All Might.
Katsuki felt betrayed during their first training session when Izuku activated his new quirk during the ball throwing portion. He believed that Izuku had kept his quirk secret since they were bakudeku kids. Aizawa had to restrain him as he charged bakudeku at Izuku and demanded answers. Izuku, having promised All Might he would keep One for All secret, had to leave Katsuki in complete darkness about the situation. This unknowingly made Katsuki more upset at being mocked and betrayed his childhood friend.
After the Hero vs. It shows Katsuki’s guilt when he realizes that he went too far in the Hero vs. All Might fight and how badly he injured Izuku the first time they fought. Izuku awakens at the nurse’s bed and goes to his classroom to see that Katsuki has left. He rushes to find him despite his friends telling him not to. Izuku confronts Katsuki, assuring him that he is not deceived. He also reveals that his quirk was a “borrowed ability”. However, he decides to not mention All Might’s involvement. He declares that he will make the borrowed power his own, and surpass Katsuki in strength. Katsuki is frustrated and cries out to Izuku, even though he doesn’t understand what Izuku meant. He yells at Izuku not to give up, but to train hard, determined to be number one. We see Katsuki openly talking to Izuku, despite their past. This theme is continued by One For All.