Mei Meido is a central character who serves as the eighth girlfriend of Rentarou Aijou. She is the live-in maid for the wealthy Hanazono family, attending to the matriarch Hahari Hanazono with unwavering dedication.

Her most immediately recognizable trait is her physical appearance. Mei has neatly styled black hair, typically tied up in a chignon or braid and covered by a white headcloth, and she wears the traditional black and white uniform of the Hanazono estate. Her most striking feature is that she is almost always seen with her eyes closed and a serene, gentle smile on her face. This is not a stylistic choice or a joke, but a physical condition caused by years of maintaining that expression of readiness and service. The muscles around her eyes have become locked, rendering her functionally blind and unable to open them at will. Despite this, she navigates her duties and the world around her with seemingly no difficulty, perceiving her environment through sound, presence, and what she calls a "mind's eye". On the rare occasions when she is subjected to extreme emotional shock, such as surprise or despair, her eyes will involuntarily snap open, revealing that she possesses irises containing all the colors of the rainbow.

Mei's personality is defined by a quiet, formal, and deeply reserved demeanor. She is unfailingly polite, using honorifics when addressing Rentarou and the other girlfriends, and speaks in a humble, respectful manner. Beneath this calm exterior, however, lies a character of terrifying intensity and fanatical loyalty. Her entire sense of self-worth is predicated on her ability to serve and follow orders, a mindset that stems directly from her tragic background. She was abandoned by her parents, who were consumed by debt and domestic violence, and was left to collapse and die during a harsh winter. She was discovered by Hahari Hanazono, who rescued her and gave her a home, an act that became the cornerstone of Mei's entire existence. Consequently, her devotion to Hahari is absolute and all-consuming, bordering on religious worship. She genuinely believes that her life has no value beyond being useful to her master and will gladly offer to "eliminate" anyone she perceives as a threat to Hahari, Rentarou, or the rest of the family. This loyalty manifests in darkly comedic ways, as she is frequently shown attempting to punish herself for perceived failures by seeking out a scalpel to cut her eyelids open, pouring corrosive chemicals on herself, or offering to donate her vital organs as repayment. Her happiness is so secondary to her own consciousness that when asked what it might be, she earnestly guesses, "A rock on the roadside?".

Her primary motivation is to serve and protect the Hanazono family, a mission that expands to include Rentarou and the entire Rentarou Family after she becomes a girlfriend. She initially resisted her feelings for Rentarou, believing it to be an unforgivable betrayal to fall in love with the same person as her master. However, the moment Hahari gave her permission to be honest with her feelings, she immediately confessed and asked Rentarou to date her, demonstrating that even her personal desires are subordinate to the commands of the person she serves.

In the story, Mei plays a multifaceted role. She is the ultra-competent aide who manages the logistical and domestic chaos the large family often generates. Her abilities as a maid are nothing short of superhuman. Despite being unable to see, she excels at all household chores including cleaning, cooking, and etiquette. She is a superb chef, known for making excellent tea and preparing bento boxes filled with Rentarou's favorite egg dishes. Her skills, however, extend far beyond the domestic sphere. She holds licenses to drive cars and pilot helicopters and private jets, all of which she does with her eyes closed. She is also a formidable combatant, possessing marksmanship skills, self-defense techniques, and superhuman athleticism. During a baseball game, she filled in as a pitcher and threw bizarre "magic balls" of the kind seen in parody sports manga. When Hahari gives her a direct order, her abilities are further enhanced, a phenomenon Rentarou compares to a character being buffed in a video game.

Her key relationships are few but intensely significant. Her entire world revolves around Hahari, who is both her savior and the sole object of her worship. Her relationship with Rentarou is built on the conflict between her extreme servant mentality and his unwavering belief that everyone has inherent value. He is the first person to tell her that she is helpful by her very existence, and that she does not need to earn the right to be loved by taking orders. Through his affection, she slowly begins to accept that it is acceptable to be cared for, rather than only to serve. She also has a significant relationship with her junior maid, Mai Meido, whom she treats with professional strictness, not hesitating to scold or even administer a spanking when necessary, though this stems from a place of senior guidance rather than cruelty.

Mei's development throughout the series is a gradual process of learning that she has intrinsic value. While she never loses her core identity as a fiercely loyal and self-sacrificing maid, Rentarou's love helps her to start conceptualizing her own happiness and to accept the devotion that others, particularly Rentarou, want to give her in return. She begins to experience the joy of being served and treasured, a feeling that was utterly alien to her before their fateful meeting. Her journey is about learning that her life has meaning beyond the orders she is given.