Mey-Rin is the maid of the Phantomhive household. She is a young woman with hazel eyes and straight maroon-red hair that she typically wears tied in high pigtails. Her most distinctive feature is a pair of large, thick, round glasses, which were a gift from Ciel Phantomhive and which she refuses to replace despite their poor fit. She suffers from extreme farsightedness, and her glasses do little to correct it properly. This condition makes her notoriously clumsy in her daily chores: she adds too much soap when washing clothes, trips and breaks fine china, and generally creates chaos whenever she attempts a task. When given an important duty, she stutters and panics, which only amplifies her awkwardness. Her clumsiness intensifies in the presence of Sebastian Michaelis, toward whom she harbors a strong romantic infatuation.

Before joining the Phantomhive staff, Mey-Rin was a highly trained assassin and an expert sniper who worked for a dangerous organization. She was once assigned to assassinate Ciel, but Sebastian intercepted her attempt. Instead of killing her, Sebastian offered her a chance to start a new life as a maid. She accepted and has since served the household with genuine devotion. Despite her ineptitude at domestic tasks, her past skills remain intact. She is an extraordinarily accurate marksman, capable of handling a variety of firearms, including a sniper rifle that she wields with ease due to her sharp farsighted vision. She does not use a telescopic sight, relying instead on her natural eyesight, which is exceptional when she removes her glasses. She is also physically fit enough to use a heavy rifle like a handgun and can rapidly reload multiple weapons.

Mey-Rin’s personality is enthusiastic and sweet-natured on the surface, which masks the competence and lethal precision she possessed as an assassin. She is easily flustered, especially around Sebastian, and her nervousness often leads to comedic accidents. However, her core motivation is deep gratitude and loyalty to Ciel, who gave her a purpose and a home. She is determined to fulfill her role as a maid and protect the family, even if her methods are unorthodox. She works alongside the other servants, Bardroy and Finnian, and shares a camaraderie with them, though she is often the source of lighthearted mishaps. Her relationship with Sebastian is marked by her obvious crush, which he largely ignores, and by a professional respect for his abilities.

Mey-Rin undergoes notable development as her past is gradually revealed. In the anime, her history as a sniper comes to light during a key episode, which adds depth to her character and recontextualizes her clumsiness as a deliberate or accidental disguise for a dangerous past. In later arcs, she actively uses her combat skills to aid the household, demonstrating that she is far more than a bumbling servant. Abilities such as her phenomenal aim, her ability to shoot from great distances without a scope, and her physical endurance make her a valuable asset when the Phantomhive family faces threats. Her loyalty and willingness to fight for those she serves define her growth from a former contract killer into a devoted member of the household.