Reiko Yamaguchi serves as a military analyst for the Japanese Defense Agency. She spearheads the investigation into the newly emerged volcanic island "New Autumn Island" during an official mission aboard the JMSDF flagship Kongou. Her duties involve reporting findings and coordinating the military's response to unidentified threats, including a reconnaissance robot and a mysterious young man found on the island.

She is the aunt of Harumi Yamaguchi and sister to a hospital director. After the young man, identified as "Mars," is rescued, Reiko takes him under her supervision at a hotel. She tasks her niece Harumi with interacting with Mars to evaluate his communication skills. However, Mars is abducted by an entity named Ra while under her watch, prompting Reiko to intensify her involvement in the escalating crisis involving hostile entities known as the "Six Gods."

Reiko collaborates closely with Ogaki, a system engineer for the Defense Agency, especially during military engagements against the Six Gods, such as the battle with the entity Sphinx. Her role focuses on coordination between defense forces and strategic decision-making. She also arranges for Mars' temporary stay with her brother's family to facilitate his research into humanity, though this subplot remains underdeveloped.

Her character arc includes encouraging a potential relationship between Harumi and Mars, reflecting a narrative enhancement over the source material. However, this arc, along with her professional and familial roles, remains unresolved due to the discontinuation of the OVA after two episodes, leaving her overall development and deeper personal backstory unexplored.