He is one of the series' main characters and commonly acts as a de facto antagonist, since several episodes involve Yukinari, Miharu and Kirie trying to overcome the plots that Fukuyama sets up to get something out of them (most commonly sexual favor from Kirie).
He is considerably perverted and goes to great extent and great expense to get close and do perverted things to Miharu and Kirie, particularly the latter. He has been known to organize catfight tournaments, using black magic and several other schemes to get to see Kirie nude, only succeeding once in a plot involving a complex board game tournament, which resulted in Kirie losing and having to give him a sponge bath with her breasts. he has a deep obsession with Kirie's breasts, to the point that he keeps a shrine to them in his closet.
Fukuyama usually antagonizes Yukinari. He first appeared in the second episode, when he first learned about Miharu, who came to Earth, and her presence at his school, he becomes crazily excited and sweeps her from her feet, rushing over to every girl's bathroom to show Miharu, puzzling her with who he is. When Yukinari tried to save Miharu from him, Fukuyama fooled Yukinari into opening girl's loockeroom, who believed were in there, only to be beaten up by girls who were changing after Yukinari saw them. Fukuyama then appeared before Kirie, who was watching Yukinari get beaten up, telling her Yukinari is a public enemy to womanhood. When Kirie was about to strike him, Fukuyama pushed her aside and wiped her unconscious. Miharu worries about Yukinari getting beaten up, but Fukuyama tells her to forget about Yukinari and to stick with him in his perverted wonderland. He then began laugh maniacally until Miharu caused an explosion, knocking out Fukuyama and the girls beating up Yukinari. As Fukuyama wakes up shortly after, he comes face to face with an angry Kirie, much to his horror. He tried to apologize to her, but Kirie punches him into the boy's lockeroom, where the boys beat him up for appearing before them while changing.
He tries to do kinky things to Koyomi but Kirie usually stops him by hitting him over the head.
Despite all this he not such a bad guy. He is very protective of his younger sister and worries when he can't find her. He strongly disproves of Lisa's attraction to Yukinari. When he first meets Tomo, he realizes she is lost and is very kind and compassionate when taking her back to Yukinari's house. He continues to help her when he sees her, and the two eventually develop a sibling-like relationship and often teams up with her. He generally acts with contempt towards Yukinari but does act friendlier (ie not constantly insulting him) towards him eventually. He has often been showing to have genuine feelings for Kirie and in the last episode is willing to injure himself to protect her.