Aquarius

"The Water Bearer" is a Celestial Spirit that is one of the 12 Golden Zodiac Keys.

Aquarius is a moody and ill-tempered Celestial Spirit who often threatens to kill her owner, Lucy. When summoned, Aquarius will either attack the opponent, Lucy, or both of them, depending on her mood. Despite this, Aquarius does care for Lucy's well-being, which will later be elaborated on.
Aquarius' previous master had been Lucy's mother, Layla Heartfilia. When Layla retired from her work as a Mage because of her health problems, she entrusted Aquarius' Key to another Mage with instructions to present it to Lucy if she chose to be a Celestial Spirit Mage. Aquarius' key was the first key that Lucy ever received...Because there weren't any other children to play with, Lucy would summon Aquarius as a playmate. This greatly infuriated Aquarius, however, learning why Lucy summoned her, the Celestial Spirit couldn't help but feel bad for the girl.