Takashima Kirihiko

An old acquaintance of Natsuki from high school. He is in a way his drinking partner and his confidant.

He serves as a model for Tsukumo's story of his sister's fake boyfriend.

He apparently met Tsukumo once in the past when he was younger - but Tsukumo is sure he never met him before - and he also adds that they were the same age at the time ...

Spoiler

It is thanks to him and his research that Tsukumo knows the cause of Natsuki's death.

He has the same last name as Takashima Kimihiko from PurexCure and Takashima Haruhiko from Koibumi Romantica; the three of them also have a similar appearance, personality AND occupation.