WARNING: The following contains spoilers for Season 2, Episode 11, of Kaguya-Sama: Love is War, now streaming on FUNimation.
Yu Ishigami has been a quiet background character until now in Kaguya-Sama: Love is War, and it's because he's been carrying some serious baggage from his traumatic middle schools days.
Even as a middle school student, Yu was rather reserved, but back then, he was willing to stick his neck out for others, and he cared enough about other people to have crushes. A classmate name Kyoko Otomo caught his eye after she was nice to him, and he tried to protect her from her sleazy boyfriend. But this plan backfired, and continued to haunt Yu until the most recent episode of Kaguya-Sama.
Shielding The Girl
Yu was a loner in middle school, content to be ignored by everyone. Until, that is, his kind classmate Kyoko paid him some attention. He was drawn to her, and while he was still working up the courage to do something about it, Kyoko landed herself a boyfriend: Ogiri, a member of the theater club. Yu saw them together and decided "It's meant to be," but there was more to it than there first appeared to be.
Yu walked in on Ogiri arranging a meeting with a side girlfriend. Yu realized that Ogiri was no-good and taking advantage of Kyoko. So, Yu cornered him in a classroom and told him, "Quit cheating," but didn't plan to alert Kyoko or make a fuss. Ogiri defied him, and in fact tried to bribe him: Yu could have some fun with Kyoko in exchange for keeping silent and staying off Ogiri's case. Outraged, Yu started punching him, and a crowd gathered to watch the beating.
This was Yu's chance to explain everything, but no one believed him, and Yu's classmates, including Kyoko, took Ogiri's side. Yu was suspended and told to write a letter of apology. However, Yu refused to do so and kept focus on getting back at Ogiri.
Snapping Out Of It
In the present, Yu is reluctant to help with the relay race, since he thinks that helping other people is bound to hurt him, like it did in middle school. But he's wrong; Miyuki Shirogane extends a hand of friendship to Yu just before high school starts, and Miyuki empowers him. During the relay race, Yu spots Kyoko herself, and he's not hung up on her any longer. He is now running to be independent, and he very nearly wins the race despite being a substitute runner. His teammates applaud him for his valiant effort, and he's deeply moved. He realizes ordinary people can indeed be wonderful beings, a lesson that Kaguya Shinomiya also recently learned.
Kyoko, for her part, is satisfied to see Yu again and has fun sticking it to him, much to the chagrin of her friends. Kaguya hints to Ai that she invited Kyoko there to give Yu a chance to confront his past, showing her devious side at work. Overall, Yu was burned badly when he stood up for a girl in middle school, but now, he has real friends who've helped him work through his baggage.