Yoshiki Kusanaji is a young man Sumire met one day in a park while he was reading a book. After a few more meetings together (in the same park) while exchanging books to read, Yoshiki starts to develop romantic feelings for Sumire, and she for him.
He is nice, but doesn't tell Sumire the whole story about him, including lying about having a cell phone.
Yoshiki has black hair and dark grey eyes. He is taller than Sumire.
Sumire first encounters Yoshiki in Chapter 7, when she stumbles upon a park while walking. She goes into the slide tower and finds Yoshiki sitting inside with a book. In the manga, Aion is a sorcerer who possesses Lucif and later Yoshiki.
In the anime it is later discovered that he is actually one of Lucif's followers named Aion, he tries to take Sumire's fragment but stops because he truly cared for her, thus, he betrays Lucif, but Lucia, not knowing of his betrayal, kills him.