Dark Mercury, portrayed by Chisaki Hama, has a markedly different uniform from her "good" counterpart.
In Pretty Guardian Sailor Moon, Ami is briefly taken over by the power of the Dark Kingdom and becomes Dark Mercury. This form first appears in Act 21, as a servant of Kunzite. Her sailor fuku has black tulle and lace on the back bow and sleeves, and tribal designs appear on her tiara and boots. She also gains a chain with a black charm on it around her waist. Her transformation phrase is Dark Power! Make-up! and is said in a much darker tone. She wields a sword fashioned from an icicle, which she creates herself the first time her transformation is displayed. In promotional photos prior to her premiere, she was shown with a different sword, which seemed to have strings on it like a harp or violin; the latter seems more likely, as she is also seen holding a bow. This bow was redecorated and given to Zoisite for use as a sword.
Dark Mercury is created when Kunzite manages to kidnap Ami in a moment of vulnerability while the other Senshi are busy. He exposes her directly to the power of Queen Metaria, causing drastic personality changes as well as the alterations to her uniform. She is self-confident to the point of egotism, and continues to attend school in civilian form, mainly to antagonize Usagi by brainwashing all of her former friends. Dressing predominantly in black, evil-Ami tends to move about slowly and dramatically, and when confronting the Senshi gives a sense of sadistic glee.
Dark Mercury has no intention of being a follower to anyone, and is always trying to pursue her own agenda, which is to kill her friends and become as strong as possible. She shows blatant disrespect to Kunzite and the other Shitennou, even to Queen Beryl, perhaps because, unlike even the Shitennou, she had been directly exposed to Metaria's power during her conversion, as opposed to having Queen Beryl or another intermediary filtering it.
Despite these alterations, certain aspects of the real Ami still remain. She still wants to do well in school, and wants friends, hence the brainwashing of her classmates. She seems to retain a sense of sympathy, which is evident when she repairs Nephrite's cape for him, stating that she does not like to see him alone.
Periodically, Sailor Moon attempts to heal her friend with the power of the Silver Crystal. Mercury is always snatched away before this can be completed, but it has some effect, ultimately resulting in her recovery. In Act 28, the catalyst for her finally returning to normal is when, having defeated Sailor Moon in battle, the sight of her injured friend causes her to realize she cares about Usagi, and to remember who she really is. After being healed, she has no memories of what happened while she was Dark Mercury. This haunts her, as she becomes terrified of what she may have done to her friends while not in control.