Miia is a lamia, a being with the upper body of a human and the lower body of a snake. She has long bright red hair, amber eyes with slitted pupils, large pointed red ears, and small scales on her cheeks. Her most prominent physical feature is her long, red serpentine tail. As the first liminal girl to take part in a government cultural exchange program, she becomes a ward of the human protagonist Kimihito Kurusu and lives in his home.

Miia is sweet, naive, and carefree by nature, but she also possesses an active, passionate, and sometimes reckless temperament. She is deeply affectionate and overtly romantic, often calling Kimihito darling and openly pursuing him as her partner. Her devotion can manifest as clinginess, especially when she sleeps, and she is known to wrap her tail around him instinctively. She is also highly possessive and jealous when it comes to his attention from other monster girls. A notable quirk is that the tip of her tail is extremely ticklish, a weakness that Kimihito sometimes uses to escape her grip.

Her primary motivation is to build a happy life with Kimihito and to be fully accepted in human society despite her hybrid nature. She approaches even strict interspecies regulations with an optimistic belief that love can overcome any obstacle. She is a hopeless romantic and wears her heart on her sleeve.

As a lamia, Miia possesses a number of abilities tied to her snake-like physiology. Her tail grants her exceptional flexibility and the ability to constrict objects or people with considerable strength. She is an adept climber and a strong swimmer, and her tail allows her to move quickly and silently. However, she is a poikilotherm, meaning her body temperature is dependent on the environment; on cold days she becomes sluggish and cannot move well if her body temperature drops. She is also catastrophically bad at cooking and housework, though she earnestly tries to improve for Kimihito's sake.

Miia's central relationship is with Kimihito, who is her host, protector, and the object of her affections. She also interacts closely with the other monster girls living in or visiting the household, including the harpy Papi, the centaur Centorea Shianus, and the arachne Rachnera Arachnera. These relationships range from friendly camaraderie to fierce rivalry for Kimihito's attention. She also has interactions with Mrs. Smith, the government agent who oversees the exchange program and serves as both a guide and an authority figure.

Over the course of the story, Miia develops from a girl who is initially insecure about her place in the human world into someone more confident and proactive. She learns to balance her possessive instincts with coexistence, and she grows more willing to protect Kimihito and her fellow monster girls. Her journey involves embracing her uniqueness rather than hiding it, and she gradually asserts a stronger role within the household.