Harumi is a young Japanese girl whose full name appears in some sources as either Harumi Yokoyama or Harumi Yamaguchi, reflecting inconsistencies in available records. She is the niece of Reiko Yamaguchi, a military analyst working for the Japanese Defense Agency, and the daughter of Reiko’s brother, who serves as a hospital director.

Following the rescue of the amnesiac young man known as Mars from the newly formed volcanic island called Heisei, Reiko takes charge of the situation. To better understand this mysterious individual, Reiko assigns Harumi the specific task of interacting with Mars. The purpose of this assignment is to evaluate Mars’s communication skills and his ability to relate to human beings, placing Harumi in a direct but controlled relationship with the protagonist.

Harumi’s role in the narrative is primarily as a link between the ordinary human world and the extraordinary circumstances surrounding Mars. Her interactions are intended to facilitate his exploration of humanity, though this character arc was not fully developed due to the early conclusion of the original video animation series after only two episodes. Consequently, detailed aspects of her personality, personal motivations, and any notable abilities remain unspecified within the available material. Her key relationships are defined by her familial connection to her aunt Reiko and her designated, observational association with Mars.