The older twin brother of Haruka Hosokawa. Although he seems to be harsh and full of hatred when he first met Hotaru, he is actually kind-hearted. He also can easily get embarrassed as well when he is with Tachibana or his brother, Haruka. It can be seen that he's a tsundere. Haruki displays an inherent hatred towards Hotaru and the team members from Toy☆Gun Gun. Despite still being active in Survival Games, he prefers to keep his identity hidden such that know one else recognizes him.

Spoiler

Haruki was recruited as the third member of Toy ☆ Gun Gun, but resigned before Hotaru joined. In the past, Haruki felt that Masamune only cared about beating Team Hoshishiro and getting back at Nagamasa, thus steps down from the team. Afterwards, Haruki feels as if he has been replaced by Hotaru and how he acted towards Masamune was unforgivable; however, Masamune kindly accepts him as the team's fourth member again.