Played by: Chisaki Hama
First appearance: Act 1
(Note: She only appears briefly in act 1, and this page contains spoilers for act 2)

Ami and Usagi are in the same class at Juuban Middle School. In many ways, Ami is the exact opposite of Usagi. She always gets to school early, she routinely gets the best test marks in the school... and she doesn't have any friends. However, Usagi can't help but try to make friends with her, and she discovers their common interest - Minako Aino.

Soon after this, it emerges that Ami is the second Sailor Guardian, Sailor Mercury. Ami is reluctant at first - she says she has too much work to do for school and juku (cram school) - and thinks that the only reason Usagi wants to be her friend is because she's a Guardian. Despite Luna's insistence that she has to fight, Usagi doesn't want to force her, but when she and Usagi are in danger Ami changes her mind anyway. Sailor Mercury, a dedicated Sailor Guardian, attacks enemies using the power of water.

Ami's mother is a doctor who apparently doesn't have much time to spend with her. Ami's ambition is to become a doctor so that she can live up to her mother's expectations.

When Ami becomes Sailor Mercury, her hair changes from black, straight and reasonably long to pale blue and quite short. Also her glasses disappear.