In her first appearances, Mercury creates an ice-cold fog that helps Sailormoon defeat the youma that took over Crystal Academy and tries to use it against Jadeite. He freezes it. This attack doesn't seem to have a name. Perhaps it's supposed to be Shabon Spray (because it doesn't seem to be in the manga, named anyway). Luna calls her "water warrior" despite her mostly ice-related attacks. I suppose that this name makes Neptune the "sea warrior".
During the Silver Millenium, Mercury was the princess' tutor. Supposedly she and Zoicite were paired up. Unlike Venus and Kunzite, there was not even a hint of this pairing in the story. She was the Princess of Mariner Castle, named after the first satellite sent to the small planet because Mercury has no moons. The same was done for Venus.
Mercury's powers, as I'm sure you've heard a million times before, are 80% defensive. She and Saturn share that. Merc's most powerful defensive attack is probably "Hyperspace Area Formation" which is a manga-only attack. It protects the arcade (well, mostly) from the battle in which Venus kills Beryl.
Play by play:
In the R season, Mercury was captured by Beruche and Rubeus, and of course escapes with the help of Sailormoon.
In the S season, she went up against Viluy, and her downfall later was the illusion of a huge supercomputer and artificial intelligence.
In SuperS, her opponent was Fisheye, who played upon her feelings of loneliness and incompetence compared to her fellow Senshi.
In StarS, Mercury's Sailor crystal was taken second with Jupiter's by Aluminum Siren.
Fact: Mercury pushes her right earring for her visor to pop out.
Fact: Kanji for Mercury is "water star".
Fact: Ami received her Mercury pen from the Sailor V game after she got the top score.
Fact: (anime) Mercury slaps Sailormoon so she would stop crying over Jupiter's death. She then tells the remaining three senshi that she must stay behind as a decoy because she is the weakest senshi.
