She is voiced by Eri Kitamura in the Japanese version of the anime and Jessica Cavanagh in the English version.
Aquarius resembles a royal blue mermaid. She has a long, blue fish-tail, light blue hair that reaches up to her waist, and a large bust. She has blue eyes that seem to have no pupils. Aquarius wears a revealing dark blue bikini top and on each arm, she has a golden armlet and bracelet. Aquarius also has a headband, three piercings on her tail, and a golden belt at her waist and light blue crystal earrings. A dark blue tattoo resembling the Aquarius zodiac symbol rests right under her collarbone. Aquarius also wields an urn that acts as the basis of the Water Magic she employs in battle. This design reflects her respective zodiac sign; "Aquarius" is often depicted as a merman or mermaid carrying a vessel from which a pouring stream of water emerges. As a swimming instructor, she takes a complete human form.
Aquarius is a moody and ill-tempered Celestial Spirit who often threatens to kill her owner, Lucy. While other Celestial Spirits serve Lucy obediently, Aquarius is often irritated by her. When summoned, Aquarius will either attack the opponent, Lucy, or both of them, depending on her mood. She is also very peculiar about where Lucy summons her, which unlike other Spirits has to be a body of water of some kind. The places she dislikes being summoned from include fish bowls, toilets and Juvia's body, though she is shown to get along with the latter, likely due to the latter being a Water wizard.
Despite this, Aquarius does care for Lucy's well-being. She is very kind and loving towards her boyfriend, Scorpio, to whom she shows a completely different side of herself. Additionally, she threatens Lucy to keep quiet about her moodiness in front of Scorpio, lest Lucy become a "drowned corpse." When Lucy is in her presence, Aquarius tends to make remarks about having a boyfriend since Lucy doesn't have one. As time progresses, however, Aquarius' attitude toward Lucy becomes more lenient.