So in short After Sihoo and Yesung met for the first time they started to have a sunbae and hobae relationship because even if they take the same classes Sihoo is still one year older than Yesung. Then Yesung got to know Taehyuk too but they both didn’t like each other. Taehyuk is the one who spread the rumors about Sihoo‘s psychological problems even though they were actually true that’s why Sihoo probably took one year of absence. When Yesung heard Taehyuk speaking to his friends he hears about the fire for the first time. Sihoo and Yesung didn’t have any special relationship other than being Sunbae and Hobae. Yesung is unfortunately lying to be able to have Sihoo for himself.
So in short
After Sihoo and Yesung met for the first time they started to have a sunbae and hobae relationship because even if they take the same classes Sihoo is still one year older than Yesung. Then Yesung got to know Taehyuk too but they both didn’t like each other. Taehyuk is the one who spread the rumors about Sihoo‘s psychological problems even though they were actually true that’s why Sihoo probably took one year of absence. When Yesung heard Taehyuk speaking to his friends he hears about the fire for the first time. Sihoo and Yesung didn’t have any special relationship other than being Sunbae and Hobae. Yesung is unfortunately lying to be able to have Sihoo for himself.