Ami Mizuno is a shy, intellectually gifted Japanese teenager renowned for exceptional academic achievements, consistently ranking at the top of national exams. Her reserved nature, stemming from high intelligence, was initially mistaken for arrogance by peers, leading to social isolation before she befriended Usagi Tsukino. She resides with her busy physician mother, Saeko Mizuno, following her parents' divorce; her painter father maintains only limited contact via annual birthday cards. Aspiring to become a doctor like her mother, Ami excels in mathematics and science. Her hobbies include reading, swimming, chess, and listening to music, coupled with a secretly cherished and embarrassing fondness for romance novels and pop culture. She exhibits an unusual allergic reaction to love letters, breaking out in a rash upon receiving them.

As Sailor Mercury, she is the second Sailor Guardian recruited, serving as the strategist for the Inner Guardians. Her powers originate from the planet Mercury (Suisei, 水星), granting her hydrokinesis, atmidokinesis (mist/fog control), and cryokinesis (ice/snow manipulation). Early techniques like "Shabon Spray" or "Mercury Aqua Mist" create obstructive fog, while evolved attacks including "Shabon Spray Freezing," "Shine Aqua Illusion," and "Mercury Aqua Mirage" inflict direct damage using ice or high-pressure water. She employs a visor summoned via her earrings for tactical analysis and a handheld computer for data processing. Her transformations progress from "Mercury Power, Make Up" (base form) to "Mercury Crystal Power, Make Up" (Super Sailor Mercury) and "Eternal Sailor Mercury" (final manga/Crystal form), each enhancing her uniform and abilities.

Her character evolves from insecurity to confidence. In the 90s anime, she initially lacks offensive power but gains it in later seasons, displaying increased assertiveness in battle. The manga and Sailor Moon Crystal depict a more analytically rigorous approach, while the live-action series (PGSM) introduces "Dark Mercury" (Akumi), a brainwashed antagonist manipulated by Kunzite. Musicals often highlight her internal conflict between studying abroad and staying with the Sailor Guardians. Notable arcs involve her sacrifice against the DD Girls in the Dark Kingdom arc (later revived), captivity by Berthier in the Black Moon arc, and receiving the Mercury Crystal to unlock her Super form in the Dream arc.

Interpersonally, Ami forms a close bond with Usagi, who helps her overcome shyness. Romantic subplots involve mutual affection with psychic classmate Ryo Urawa before his abrupt departure in the anime. In Sailor Moon Sailor Stars, she interacts with Three Lights member Kou Taiki, though ideological differences limit deeper connections. A canonical Naoko Takeuchi illustration pairs her with Zoisite, explored musically in songs like "Itsuwari no Forevermore." PGSM features nuanced moments with Nephrite, including gifting him a cape. Alternate futures, like the "Parallel Sailor Moon" side story, depict her married with a daughter (Ami Jr.), working as an underpaid physician.

In Sailor Moon Cosmos, Ami, as Eternal Sailor Mercury, fights alongside the guardians against Sailor Galaxia and Shadow Galactica. Sailor Aluminum Seiren kills her, stealing her Sailor Crystal. Her physical form disintegrates, and her crystal is discarded into the Galaxy Cauldron. The cauldron's restoration by Sailor Moon via the Lambda Power revitalizes her Sailor Crystal, returning her to life alongside the other guardians.