Ami Mizuno, known as Amy in some English adaptations, is the civilian identity of Sailor Mercurius, one of the primary Sailor Guardians. She is a teenage Japanese schoolgirl who attends Azabu Jūban Junior High School alongside the series' protagonist, Usagi Tsukino. Her birthday is September 10th. Ami lives with her mother, a busy doctor who is often away from home working, and whom Ami greatly admires and hopes to emulate. Her parents are divorced, a situation Ami sometimes resents; her father, a painter, left home to pursue his art and rarely visits, though he sends postcards on her birthday.

Ami's most defining characteristic is her extraordinary intelligence, which is so pronounced that her classmates rumor her to have an IQ of three hundred. However, her brilliance and inherent shyness are often misinterpreted by her peers as coldness or snobbery, which has made it difficult for her to form friendships. In truth, she is a kind, gentle, and loyal person who is somewhat insecure and dislikes conflict. She is deeply caring and has a strong aversion to harming innocent people. While she is sensible, calm, and often serves as the voice of reason for her more impulsive friends, she is also quietly dedicated to her studies, spending most of her free time reading and preparing for her future. Her dream is to become a doctor, like her mother. Beyond academics, she enjoys swimming, playing chess, and has an appreciation for art and even pop culture, though she is often embarrassed to admit to the latter.

In the story, Ami is the first Sailor Guardian to be discovered and befriended by Sailor Moon after Luna, the talking cat, identifies the mark of Mercury on her forehead. Her primary motivation is to use her abilities to fight evil and protect others, all while balancing her duties as a Guardian with her ambitions as a student. She is fiercely loyal to her princess, Sailor Moon, and is willing to sacrifice her life for her. Her key relationships are with her fellow Guardians, especially Usagi Tsukino, who was the first person to see past her shy exterior and become her true friend. This friendship helps Ami grow more confident and realize there is more to life than just studying. She also shares a close bond with Makoto Kino, Rei Hino, and Minako Aino, forming the core team of inner Guardians.

Throughout the series, Ami undergoes significant development. Initially a lonely bookworm, she blossoms into a confident young woman who learns to value her friendships as much as her academic achievements. At one point, she is given the opportunity to study abroad in Germany but ultimately decides to stay in Japan, recognizing that her mission as a Guardian and her bonds with her friends are more important. In the live-action series, she is briefly brainwashed by the Dark Kingdom and transformed into the malevolent Dark Mercury, showcasing a dramatic and temporary shift in her character.

As Sailor Mercurius, she is the Guardian of Water and Wisdom, and her abilities are predominantly water-based. Her powers often serve a strategic purpose, reflecting her role as the brains of the group. Her initial ability is Shabon Spray, which creates a thick, chilling mist to blind and disorient enemies, giving her allies an advantage. As she grows stronger, she acquires more offensive attacks. Her signature offensive technique is Shine Aqua Illusion, a powerful blast of water that can also freeze targets solid. Later, she attains the ability to transform into Super Sailor Mercurius, which grants her the Mercury Harp, a magical lyre she uses to perform her ultimate attack, Mercury Aqua Rhapsody. In addition to her water-based magic, she possesses a number of technological tools, including a miniature supercomputer and Mercury Goggles, which she uses to analyze enemies, find their weak points, and devise battle strategies.