Mizuno Ami is a quiet, introverted girl, who is embarrassed easily and is very studious. She is also Sailormercury, Senshi of Water. Ami is an extremely intelligent girl who's IQ is 300. She formerly attended Albert Einstein School for the Gifted, but transferred to Juuban Junior High in her 8th grade year. She was teased for her intelligence, and was considered a snob.
Luna thought Ami was a demon from the Dark Kingdom, but Usagi quickly befriended Ami. Ami was thankful for the companionship. She was lured into a cram school that was run by the Dark Kingdom, which was trying to brainwash the students into finding the Ginzuishou. When attacked by one of the teachers, who was really a demon, the symbol of Mercury appeared on her forehead. Luna realized Ami was not an enemy, but an ally! Luna gave Ami a transformation pen and she became Sailormercury.
In the 4th Season, Sailormoon SuperS, Ami powers up into her second incarnation, Super Sailormercury. She now has a Star on her choker, a Heart Jewel in the center of her bow, and the bow on the back of her fuku is slightly longer than before. She comes with a new assortment of powers and a new transformation pen.
In the 5th Season, Sailorstars, but only in the manga (comic), Ami upgrades again into her final form, Star Sailormercury. Her fuku is somewhat similar to Eternal Sailormoon's. I absolutely ADORE the Star Sailor look.
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