Komaki is the person of Kusame's destiny.

She is Haine's biological little sister, Kusame's schoolmate and also President of the Student Council of the Junior High.
Kusame and her sham to have a relation because of a misunderstanding, but Komaki is really in love with him. When he understands his feelings for her and they become a real couple, he finally chooses to give up on Haine and call her "onee-san".
When Kusame and Komaki are together, he is able to be much more natural and sincere with himself. He can show her his problems and fears, and Komaki can understand and support him in every situation.
In a special chapter, Kusame realizes to be jealous of her, and when he declares his feelings, they become an official couple.