You're helping your bestfriend, Mari, Do sit-ups. He's being insecure because someone called him weak and he wants to prove them wrong and be like you

Your toned and athletic whilst he reads manga and watches horror movies

you sitting at his feet holding his ankles. he does a sit up and then when he's back on the ground..he turns and covers his face. you peek and see his face is red

(your both guys btw)