Mizuno Ami, or Ami-chan as her everyone calls her, is a genius 14-year-old girl who is a classmate of Usagi. She does not have any friends in the beginning of PGSM, and is often seen alone on the roof of the school building studying and eating a sandwich. The smart girl lives with her mother, who is a doctor, in a rich apartment.
Ami is introduced in Act 1 by Miss Haruna - the teacher tells Usagi, who is late, that the smart girl goes earlier to school than other students to read. Ami makes her first important appearance in Act 2. Usagi decided to become friends with her. She gave the genius her copy of Aino Minako's album Venus. Later Ami started listening to it at class in her cram school when a youma attacked draining the energy of all students. Ami, however, was listening to her walkman (or mp3 player), so she was not offended by the youma. When the monster saw that Ami had not lost her life energy, it started chasing her. She started running and was eventually saved by Sailor Moon's Moon Tiara Boomerang attack. While the villain was stopped, Ami had almost fell down from the school building, but Sailor Moon held her hand. Then Luna gave Ami a Star Bracelet. Ami said that she wanted to fight evil with Sailor Moon. She could not hold Moon's hand anymore, and started falling, but yelled out Mercury Power! Make Up! and landed successfully on the ground as Sailor Mercury, the Soldier of Water and Wisdom. Then Mercury used Mercury Aqua Mist, her first attack, combining it with Sailor Moon's Moon Twilight Flash to destroy the monsters.
Sailor Mercury is not a very powerful Sailor Soldier. She rarely beats the enemy herself, but she is very loyal to her friends and, as strange as it sounds, to her "rivals" like Osaka Naru. Sailor Mercury has five attacks in total which, almost always, do not cause any damage at all.
In Act 21, Ami feels really lonely because all of her friends ignore her. When she exits the Karaoke Crown, on her way home, Kunzite appears and confronts the girl. She transforms and fights, but neither Shine Aqua Illusion, nor Mercury Aqua Mist manage to take down the general. He uses his sword and blasts Ami with dark energy, then he teleports away, saying he is happy that Ami is the one who was falling in his hands.
In Act 22, Ami does not feel well and collapses while she is with Makoto in a park. The Soldier of Thunder brings her friend to a hospital, but Ami disappears. Makoto tries to find her and uses her powers to communicate with woods. She finally finds Ami, but the girl is with Kunzite. Mako-chan tries to protect her friend and fights by herself against the general, and eventually he beats her. When Jupiter tells Ami to run, she ignores her. Meanwhile, Sailor Moon and Sailor Mars arrive. Kunzite tells Ami to show her friends the new powers she possesses. She yells out Dark Power! Make Up! and becomes Sailor Dark Mercury, also known as Darkury, agent of the Dark Kingdom.
During Acts 22-28 Ami is evil and fights against the Sailor Soldiers.
In Act 28, a new Sailor Soldier (Sailor Luna) and Dark Mercury are seen on a roof of a building, ready to fight against each other. Sailor Luna tells the evil Senshi than she will be back to her normal self. Darkury is ready to attack but at that very moment she suddenly collapses and remembers something from her past as a Soldier. Sailor Luna leaves.
Later in the same episode, we learn that the plushie Luna can transform into a human. Meanwhile, evil Ami calls Usagi and tells her that she wants to meet her in a building. Usagi lies to Rei and Makoto that Ami wants to meet them, not her, so that they won't follow her. Usagi leaves the Crown Center and leaves a note to her friends Makoto and Rei. They find it later, while Sailor Moon and evil Mercury are already fighting. Usagi fights mostly protective. Mercury eventually breakes the Moonlight Stick and Sailor Moon collapses. Then Ami says, "Usagi-chan..." and gains her memory back, becoming Sailor Mercury again.
Later Ami and Usagi are transported in a forest in the Dark Kingdom by Kunzite. The smart girl does not remember anything about being Dark Mercury, but she feels guilty and as soon as she notices her dark transformation bracelet, Ami throws it away.
Meanwhile, Rei and Makoto go to the building in which Moon and Dark Mercury had fought before. There they see Ami and Usagi in another dimension. The four friends try to hold each others' hands, thus transporting Usagi and Ami back to the world.
Powers 
Mercury Power! Make-Up! - Ami's transformation phrase.
Mercury Aqua Mist! - This attack is a bit similar to "Shine Aqua Illusion" in the Anime. Sailor Mercury spins around and a mist of water appears in front of her body. Then, she releases it against enemies.
Shine Aqua Illusion! - Sailor Mercury's second attack. A stronger mist of water attacks the enemy. Sometimes the attack looks like a wave - Act 9 - and sometimes like a hurricane - Act 21.
Dark Power! Make-Up! - Ami uses this phrase during acts 22-28 to become Sailor Dark Mercury.
Mercury Aqua Storm! - Holding her Star Tambourine, Sailor Mercury strikes a water storm against the enemy.
Mercury Aqua Cyclone! - After the Act in which Mercury used Mercury Aqua Storm, this new attack appears. Sailor Mercury moves her hands in front of her chest to power-up some energy. Then, she makes her sign with both of her hands and strikes a strong wave of water.
Mercury Aqua Blizzard! - This is Sailor Mercury's final attack. It is used against evil Prince Endymion, but it was not successfull. Mercury Aqua Blizzard freezes the enemy for a second or two.
Water... Mercury... Give me your power!! - Sailor Mercury says this turning her Star Tambourine into a sword.
