Mizuno Ami (or Amy Anderson) was the first of the Inner Senshi to be found. A transfer student to Juuban Junior High School, her gaming skills caught the attention of Usagi, and her super genius caught the attention of Luna. Ami became the victim of a big bad dark kingdom plot at her cramming school, and was revealed when Usagi and Luna saw her symbol light up when she was attacked. Then they all saved the day.
Mercury is generally considered the most intelligent of the Inner Senshi, probably because she has a rumored IQ of well over 300 (!). She also tends to be more analytical and cautious about fighting. As a defensive rather than offensive fighter, her attacks (in forms of soap bubbles, water, ice, etc.) seem to be less effective than those of other senshi, even if hers do look cool.
Ami is a cute and kind character (with a really talented seiyuu (voice actress) Hisakawa Aya). She is quite shy in the beginning of the series, but as the show continues, she opens up gradually. In general, Ami provides a voice of reason when everyone else is being silly, though she does do the 3 Lights chasing thing in Stars.
Ami also has a cool little blue minicomputer - probably the best personalized PDA one could ever have