Asanuma appeared in the Black Moon arc of the manga as an underclassman of Mamoru's at school who strongly respected him. He was a supernatural enthusiast and a member of his school's Sci-Fi Club, to which he reported any UFO sightings. After he saw Mamoru heal himself and received a static shock from Makoto, he believed that the Senshi were aliens, but when he asked Makoto she denied everything. When he revealed that he heard Luna talking to Usagi, Makoto confessed that she was Sailor Jupiter, admitted the identites of the others, and kissed him on the forehead. Asanuma appeared again briefly in Act 24, the beginning of the Infinity arc, and then once more in Act 45 of the Stars arc, when Usagi asked him if he knew Mamoru's phone number in America.
In the anime, Asanuma had a very brief scene in episode 169; although he was not named in the scene, his name appeared in the credits. He called on Mamoru at his apartment when he did not attend an offline meeting of the PC Network members, but after ringing his doorbell several times without getting an answer, Asanuma left. Usagi noted after he left that he was a "suspicious guy."