|Occupation||Cram School Teacher|
|Family and Relatives||Kyomi Kasannoin (younger sister)
Nanami Kanroji (lover)
|Seiyuu||Hideki Ogihara (credited as Takezou Koike)|
Although unseen, Kasannoin is the boyfriend of Nanami. In a few routes, Nanami can confide in Sekai, Setsuna, and Hikari about her boyfriend's otaku hobbies or his enabling of his sister's brother complex.
Kasannoin plays a more prominent role as Yuuki's sempai. Like Yuuki, he is a student librarian and enjoys online games, even to the point of noticing Yuuki's absence from an MMORPG. He also mentions to Yuuki his part-time job as a tutor (which is actually a coverup for his pedophilic activities) and, in the Disappointment ending, he is mentioned to have introduced Yuuki to Kokoro.
In other routes, he and Yuuki can go on a "double date" with Hanon and Niki, an event witnessed by Hikari. His affair with Hanon can also be witnessed by Kotonoha, which can eventually lead to Nanami breaking up with him, as seen in Given Up on Everything.
Although he's still present as Yuuki's sempai, Kasannoin's role is much smaller. There is also no mention of him being Nanami's boyfriend, his affair with Hanon, or of his "tutoring".
- His surname may derive from the Kasannoin branch of the Fujiwara clan. It may also be a nod to Morokata Kazan'in, a Kamakura period nobleman and poet.
- Kyouichi's front as cram school teacher for his pedophile activities and his infidelity is similar to Tomaru Sawagoe's actions throughout the Radish Trilogy.
- In Cross Days, he is nicknamed "Kyou-kun" by Hanon.