In the second season, Syo is closer to Nanami, shown by him giving her one of his favorite DVDs, getting upset about Cecil's romantic advances towards her (along with the others), risked his life to save her from falling, and giving her one of his hats. Also, when Syo saves Nanami from falling, he pulls her into his arms and holds her as she cries due to being scared and tells her he is glad she's alright and wouldn't know what to do if anything had happened to her.
He also comes to her aid in the second season, whenever HEAVENS attempts to take her away.
In Season 4 Episode 1, after STARISH pulls her away from QUARTET NIGHT, he holds her as says that "she knows him...he'll never let go(of her)."
Natsuki is Syo's roommate, and in the anime, childhood friend. Natsuki claims Syo is as cute as Haruka. He enjoys dressing Syo up, petting him, and spending time with him. Syo often acts as if he dislikes Natsuki, but actually Syo genuinely cares for him. In the anime, Natsuki is the reason Syo is afraid of heights, after chasing him up a tower and Syo nearly falling out a window when he was 5. Natsuki affectionately calls him "Syo-chan," "-chan" being an honorific used to express fondness, closeness, and affection and is more commonly used when addressing girls, but boys can be called it as well, possibly a more "cute" way to address a boy rather than using "-kun".
Syo and Tokiya are friends and colleagues. When the secret of Tokiya being HAYATO came out, Syo was furious and felt that he had betrayed them. Later, once Tokiya returns and explains everything, professing his deep desire to continue to sing with them, Syo proceeds to hug/strangle him, claiming that he wouldn't forgive him.
Masato is a fellow member of STARISH.
Otoya is Syo's fellow STARISH member.
Syo didn't like Cecil in the beginning, since he tried to kiss Haruka. Syo was slow to accept Cecil, but eventually came around with time. The two were later put in a unit project with Ren, and clashed over the best way to execute it. However, thanks to Haruka, they managed to blend their ideas perfectly and remain friends afterwards.
Kaoru is Syo's younger twin brother who appears only in the games. Kaoru cares a lot about Syo and looks after him due to Syo's heart condition. Kaoru is shown to be very protective of Syo due to his condition and cares about his brother's happiness over his own and decides to stay at Saotome Academy to look after him. It should also be noted that Kaoru is studying to become a doctor, it's possible he's trying to become a doctor in order to find a way to cure Syo's heart condition. In the games, Kaoru is shown to be somewhat possessive of his older twin brother, saying he's the only one capable of making him happy. Syo loves his younger brother immensely, but doesn't like how protective he is of him. Kaoru affectionately calls him "Syo-chan," like how Natsuki does.
Syo has looked up Hyuge-sensei since he was a child, and aspires to be like him. When Syo gets treated coldly for being a fan by him in Season 2, he is deeply hurt and depressed. Later, Hyuga apologizes for his words, and Syo's admiration for him is rekindled.