Daigo Oguruma is a medical doctor and engineer affiliated with the Fenrir organization, who first appears in the God Eater game series. He is a man of approximately fifty years of age. His primary role is that of a physician, having served as the personal doctor for the God Eater Alisa Amiella during her time at the Fenrir Russia Branch before both were transferred to the Far East Branch.

On the surface, Daigo Oguruma presents a calm and tranquil demeanor, which allows him to operate effectively within the Fenrir hierarchy and maintain a trusted relationship with his patients. However, this exterior masks a ruthless and manipulative nature. He is deeply involved in questionable scientific and psychological experiments, secretly performing brainwashing and memory manipulation on Alisa under the guise of therapeutic treatment. His motivations are rooted in a personal history of professional rejection and a subsequent descent into fanaticism. Originally, he was a doctor dedicated to saving those abandoned during the rise of the Aragami, but after having his thesis on the relationship between the mind and Oracle cells ignored by Fenrir headquarters, he became distorted. He ultimately allied himself with Director Johannes von Schicksal to further the Ark Project, a plan for human salvation that would come at a terrible cost.

Within the story, Oguruma serves as an antagonist. Acting on orders from Director Schicksal, he is the primary cause of a critical incident where a brainwashed Alisa is triggered to attack her comrade, Lindow Amamiya. This brainwashing is activated by a specific code phrase, and involves altering Alisa's perception to view her allies as enemies. His actions directly lead to a major turning point in the plot. Following the failure of the Ark Project, he disappears from the Far East Branch and is subsequently wanted for his crimes. His quest for revenge continues; he reappears in a later epilogue novel, where he successfully develops a factor to attract Aragami and a suit for protection, using these creations in a final, failed attempt to attack the Far East Branch.

Key relationships define his role. His relationship with Alisa Amiella is central and deeply abusive; he exploits her trauma and his position of trust to control her for his own ends and those of Director Schicksal, to whom he is a subordinate. Oguruma is also an expert on New-Type God Eaters, demonstrating a level of technical and medical knowledge that makes him a valuable asset to Fenrir and a dangerous adversary. His appearance is notable for being unconventional for a doctor, often wearing a yellow bandana with a yellow shirt and jeans beneath his white lab coat. The character undergoes a minor development, transforming from a supposedly benevolent figure into an openly villainous one as his schemes are revealed, ultimately dying when his protective suit fails during an Aragami attack. In the anime adaptation, his characterization is altered to be more overtly sinister and perverted, and he attempts to manipulate events from a distance, including hacking systems to lure Aragami toward a populated area.