1 - Easily got sunburn, thats why he wears long-sleeve jersey and trousers.
2 - Hang curtain in his locker, so that no one knew what's inside...
3 - Very strict with laundry, he even wash his own clothes and iron his handkerchief! He doesn't trust his family to wash them.
4 - Likes rose patterns. His handkerchief and his seat cushion in the club room has it. In one episode, he wears a purple shirt with rose patterns on it.
5 - Being respected of his hard work in gathering data from all school, including Seigaku. He travels from one school to another just to gather data though he is easily sunburned...
6 - As everyone knows, he's Fuji Yuuta's savior. Yuuta even said, "If its not because of Mizuki, I don't know where I'll be right now.." Yes, Yuuta owes him A LOT...
7 - He is often being joke as a maid because of where he comes from...
8 - Mizuki had a habit to touch his front hair. From psychology traits, it means he's very confident of himself
9 - Surprisingly, he is actually KIRIHARA AKAYA'S LONG LOST BROTHER! No wonder Akaya got resemblance of his looks (not to mention the similarity of personalities)...