The treasurer of the student council of the Shuchiin Academy. He's extremely valued by Miyuki, given that he single-handedly manages all the school's funds. He's also terrified of Kaguya, though no one else is sure why that is. Yuu keeps to himself and rarely speaks up, often getting into trouble when he does. He has a depressed outlook on life while having an inferiority complex. In contrast to his attitude, he is willing to help those in need and will sacrifice his pride if need be. Unlike his high school self, he had a much different outlook and demeanor during middle school.
Later in the series:
Ishigami develops a crush on Tsubame that he inadvertently confesses his feelings for her at the school's cultural festival. Ishigami does not realize that his actions were perceived as a confession by the rest of the student body and is therefore oblivious to the implications of having given Tsubame a heart-shaped gift (considered akin to a confession in Shuchiin tradition). After some convincing by Kaguya, Tsubame decides to seriously consider Ishigami's confession, with her original intention being to let him down as gently as possible. It isn't until Ishigami watches Tsubame's performance of the traditional play that gave rise to the aforementioned tradition that he realizes his actions had unintended meaning. Even then, Ishigami's thickheaded nature prevented him from recognizing the full meaning of his actions: a straight confession
On the day of the third years' graduation ceremony, Tsubame finally has an answer for Ishigami's proposal. Despite Ishigami doing his best to convince Tsubame to accept him, Tsubame ultimately rejects Ishigami. Tsubame promises Ishigami that the two of them will remain friends and Ishigami sees her off with a smile and a gift of flowers. Afterwards, Ishigami breaks into tears, finally learning what heartbreak is like.
