Shiketsu High School#
Inasa Yoarashi#
Seiji and Inasa appear to have a calm and respectful relationship. Inasa looks up to Seiji and even refers to him as "senpai". Seiji has a good understanding of Inasa's personality and tries to keep him in check when he does something without dignity.[1]
Camie Utsushimi#
Seiji is easily annoyed by Camie's easygoing personality, which contrasts with his serious nature. He often yells at her angrily only to get an empty response from his classmate. She doesn't appear to listen to anything he says and this clearly annoys him.
Even so, Seiji did display concern for Camie after Himiko Toga impersonated her and was angry at himself for not noticing something was off with his classmate.[2]
U.A. High School#
Katsuki Bakugo#
Seiji and Katsuki share a mutual dislike towards one another. They're polar opposites in regards to becoming heroes. Seiji believes Katsuki is vulgar and undeserving of becoming a hero. In return, Katsuki constantly makes fun of Seiji's formal attitude by calling him "senpai". He also taunts Seiji about his name and the size of his eyes.
Even after the Provisional Hero License Exam, Seiji and Katsuki still do not get along at all.
Denki Kaminari#
Since Seiji watched the U.A. Sports Festival, he was aware of Denki's Electrification being uncontrollable and he taunts him by saying that he can't use his Quirk as it will also hurt Eijiro Kirishima and Katsuki despite their deformed state as meatballs. Denki took those insults personally, especially when Seiji stepped on Eijiro's deformed body while taunting him. He ends up underestimating Denki, which leads to him being defeated by the Stun Gun Hero with the aid of the bodily reformed Eijiro and Katsuki.