Asanuma appeared in the Black Moon arc of the manga as an underclassman at Mamoru's school who greatly admired him. He was a fan of the supernatural and a member of the Sci-Fi Club, always eager to report any UFO sightings. After witnessing Mamoru heal himself and experiencing a static shock from Makoto, he concluded that the Senshi were aliens. However, when he questioned Makoto, she denied everything. Upon revealing that he had overheard Luna talking to Usagi, Makoto confessed to being Sailor Jupiter and disclosed the identities of the other Senshi, even kissing him on the forehead. Asanuma briefly reappeared in Act 24, marking the start of the Infinity arc, and later in Act 45 of the Stars arc, where Usagi asked him if he knew Mamoru's phone number in America.
In the anime, Asanuma made a very brief appearance in episode 169, though he wasn't named in the scene, his name appeared in the credits. He visited Mamoru's apartment after Mamoru missed a meeting of the PC Network members, but after ringing the doorbell multiple times without receiving an answer, he left. Usagi remarked after he left that he seemed like a "suspicious guy."
