Mizushima, codenamed Mercury, works as a designer in the Animal Design Department of the Heavenly Creative Agency. He presents with short black hair, glasses, and a consistently practical outfit: a navy blue turtleneck, light-blue jeans, and sneakers, conveying a laidback yet focused air.

His personality leans aloof and stoic. He champions simplicity and efficiency in work and personal life, evident in his minimalist lunch of plain rice balls, water, and vitamin pills. Deep passion surfaces when defending or showcasing his masterpiece creation, the snake, often prioritizing it over other animals—particularly birds designed by colleague Kanamori. This reflects a professional rivalry grounded in biological reality, as his snake designs frequently prey upon Kanamori's avian creations.

Mizushima's design philosophy centers on functional innovation and biological efficiency. He rejects needless complexity, favoring sleek adaptations that boost survival. This approach yields designs like the flying snake, rattlesnake, and egg-eating snake, all derived from his core snake model. His broader portfolio includes the chameleon, crocodilian species, carp, electric eel, ping-pong tree sponge, and the rejected eastern dragon (deemed impractical).

He maintains professional respect for department chief Tsuchiya (Saturn) and engages in pragmatic collaboration with peers. Mizushima frequently critiques biologically implausible concepts, such as Tsuchiya's horse variants or designs violating anatomical constraints. Interactions with Kimura (Jupiter) involve tension due to Kimura's habit of consuming prototype animals, though they occasionally collaborate on projects like symbiotic species relationships.