Mamoru Sekai operates as the trusted partner of Galaxy Exalt, Cosmoman, championing peace through battles against threats such as rampaging Illegal monsters. During a pivotal confrontation, he rescues Masato Rikuo from a monstrous attack, inadvertently revealing his secret identity as Cosmoman. Though Masato initially swears to protect this secret, Mamoru proactively discloses his dual role publicly in the same episode, rendering confidentiality unnecessary.

His competitive Buddyfight career features a decisive victory over Light Kurouzu (Episode A8) and a strategic defeat by Masato Rikuo (Episode A9). The narrative underscores his unwavering dedication to heroism via Cosmoman, merging combat prowess with thematic echoes of classic tokusatsu vigilante traditions.