高島 霧彦

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 ...

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.