Milly, known as Miri in the original Japanese version, is a robot created by Dr. Kawashimo shortly before Jetter Mars's activation. Her Japanese name, 美理, combines the characters for "beauty" (美) and "reason" or "logic" (理). Identified as Jetter Mars' half-sister due to their shared creator, she possesses a distinct design and more mature personality. Initially presented as Dr. Kawashimo's daughter, her true robotic nature is revealed by the end of the first episode.

She contrasts Mars' impulsive, childlike behavior with responsibility and compassion, often acting as a stabilizing influence. Milly demonstrates a strong familial bond, accepting Dr. Kawashimo as her father and serving as an older sister figure to Mars and his younger brother Melchi. She frequently assists Mars in understanding human emotions and moral dilemmas, intervening when his actions cause unintended harm.

Her primary special ability involves restoring broken objects or machinery, often described as welding or repair, though she uses this power sparingly. Physically, she is athletic; her design typically includes pink boots, though some depictions show dark green boots.

Key events include her disappearance in the episode "Missing Miri" (Episode 7) and the exploration of her identity in "Miri, alone again" (Episode 24), where another identical robot appears. Her role consistently revolves around guiding Mars, resolving conflicts stemming from his dual nature, and providing emotional support within their unconventional family structure.