Jennifer Portman is an American scientist arriving in Japan to investigate Extended Definition, a survival mechanism causing species to transform into monstrous combat forms to prevent extinction, often triggering attacks against the entity Hikari. To observe closely, she moves in with Takeru Takemoto, Hikari, and Mari Nishino.

She blends professional brilliance with off-duty eccentricity, appearing ditzy or idolic outside work yet demonstrating high intelligence during investigations. Her personality includes frequent alcohol consumption and exhibitionist tendencies; she often carries alcohol, rarely seen without it. Characterized as a "busty foreign drunkard" and "hard-drinking party girl," she frequently coerces the robot Ioneos into drinking with her. Physically, she possesses blonde hair and petrol-colored eyes.

Her demeanor combines a commanding, assertive attitude with a strong sense of purpose, though harsh language occasionally reveals frustration or aggression, potentially straining relationships. She maintains a close professional association with the cosmic entities Hikari and Akari, studying their connection to extinction and survival dynamics while integrating into their human social circle. Her scientific role provides key exposition on the series' central conflicts, particularly regarding Extended Definition and the existential threats tied to Hikari's true purpose.