He helped his teammate to pass a love letter to Yuko Harada. After Yuko learn the truth about the letter she felt disappointed as she has a crush on Sugimura and thought he felt the same for her. She then yelled at him and ran away.
Sugimura asked Shizuku what happened to Yuko and Shizuku said he's an idiot as Yuko has a crush on him, and it broke her heart knowing that Sugimura helped his friend to pass the letter and didn't know that she liked him.
Sugimura felt shocked and apologies for his action, and he tells Shizuku that she is the one he's in love with, but Shizuku sadly rejected him and said that she only sees him as a friend and cannot betray Yuko as she is her best friend. Although upset at this confession, he reluctantly agrees to just be friends with her and never mentions it again.
Afterwards, Sugimura apologizes to Yuko and that he'll tell his friend she is not interested, while Yuko also apologizes for yelling at him, which he accepts. Later on, when Seiji Amasawa come and look for Shizuku in their classroom, Sugimura showed a sad and defended face, but keeps it to himself. He is not seen again for the rest of the film until the closing credits, where he meets up with Yuko, implying that they get together as a couple.