Gastaman is a character who appears exclusively in the Marmalade Boy Movie, which serves as a prelude to the television series. In this theatrical production, Gastaman is part of a group of children who portray heroes and engage in antagonistic behavior toward one of the main protagonists. Specifically, these children are seen bullying Miki Koishikawa while she is practicing tennis. The character is listed in supporting roles for the film, with different iterations or versions of the character appearing under names such as Gastaman Omega and Gastaman. The character exists within the context of the movie's self-contained story and does not appear in the main Marmalade Boy television series. Information regarding Gastaman's individual personality, backstory, motivations, character development, personal relationships, or specific abilities is not available as the character is primarily noted for this singular scene of antagonizing Miki during her tennis practice. The portrayal involves children engaged in imaginative play as heroes, with their actions directly impacting the protagonist in a negative manner during a recreational activity.