Mitsuki Sawatari is the 13-year-old younger sister of Izumi Sawatari, separated by one year. She and Izumi ran away from home and secured positions as maids in the mansion of 14-year-old millionaire Yoshitaka Nakabayashi. Mitsuki insisted on accommodation for her pet alligator, Pochi, as a condition for taking the job. She discovered Pochi as an egg after her parents had Pochi's parents put down following an attack, feeling responsible for their fate.

Mitsuki embraces her maid role with enthusiasm, consistently addressing Yoshitaka as "Goshujin-sama" or "Master" and openly enjoying the revealing maid uniforms, unlike Izumi. Her dedication includes public performances, such as singing the festival song "I'm a cute maid at a pervert's house!" Her popularity spawned a personal fan club spanning multiple schools.

She demonstrates sharp intelligence and strategic planning. Mitsuki accessed Yoshitaka's media archive to secretly enroll Izumi as a "Net Idol" on a website. This venture generated significant revenue, enabling Izumi to pay off her debt and inadvertently making Mitsuki wealthy from the profits. In the anime, she orchestrated events over a month to ensure Izumi would choose to stay at the mansion permanently after settling her debt.

Mitsuki frequently mediates conflicts by devising unconventional competitions, often featuring Pochi to achieve bizarre resolutions. These contests typically favor Yoshitaka's interests and compel Izumi's reluctant participation. Despite her outwardly cute demeanor, Mitsuki harbors unexpected interests like heavy metal music. Manga sources also indicate she possesses torture equipment.

Her relationship dynamics show a pattern of aligning with Yoshitaka during disputes and subjecting Izumi to teasing or sexualized harassment alongside other characters. This includes acts like flipping Izumi's skirt or attempting to remove her bra, positioning Mitsuki as an active participant in the household's chaotic interactions.