Saiki helped lead a rebellion against Saiko after he got Saiki's dad transferred to Siberia.
Saiko developed a crush on Kokomi which made him want to transfer to Saiki's high school just to be near her.
Saiko was moved when Nendou threw away money to save a kid and seems to somewhat respect him. Nendou thinks that he is still in Saiko's debt and considers himself his henchman.
Kuboyasu hated Saiko for being so smug and wanted to kick his ass.