Sōichirō Fuji is the half-brother of Marina Mochizuki, connected through their mother. His mixed Russian and Japanese heritage is evident in his blonde hair and conventionally attractive features, which often draw significant positive attention upon first meetings. Fuji is fluent in both English and Japanese. When interacting with Ichika Mizuhara and Tomoya Mizuhara, he initially inquires about their English proficiency before switching the conversation entirely to Japanese.

A defining personality trait is his significant otaku interests, heavily focused on anime, video games, and related media. This enthusiasm manifests prominently during his introduction, where he persistently approaches Ichika due to her perceived resemblance to a character from a galge he enjoys. His eagerness to discuss these niche hobbies often results in socially awkward behavior, creating a stark contrast with his initial attractive presentation that diminishes others' interest. His character background, occupation, detailed family history beyond the established half-sibling relationship with Marina, and narrative role remain confined to his introductory appearance in episode 6 across the series, films, OVAs, and related media.