Aquarius is the Celestial Spirit of the Water Bearer and one of the twelve Golden Zodiac Keys, making her a spirit of significant power and prestige. Before becoming the property of Lucy Heartfilia, Aquarius was contracted to Lucy's mother, Layla Heartfilia. It was Layla who entrusted the key to another mage with instructions to pass it to Lucy should she choose to follow the path of a Celestial Spirit Mage. Because of this, Aquarius was the very first spirit key Lucy ever received, and the spirit herself was a constant presence throughout Lucy's childhood, often being summoned to play with the lonely girl and effectively acting as a caretaker for her.

Aquarius is characterized by a famously volatile and ill-tempered personality, which stands in stark contrast to the typically obedient nature of other Celestial Spirits. She is notoriously moody and disrespectful towards her owner, Lucy, frequently insulting her, threatening to kill her, and sometimes even attacking her alongside the enemy depending on her mood. A common taunt from Aquarius is that Lucy will never get a boyfriend, a subject she uses to emphasize her own successful relationship. However, this harsh exterior is largely a facade. Beneath her irritable demeanor, Aquarius genuinely cares for Lucy's well-being and shares a deep, unspoken bond with her, acting as a true friend and protector when the situation becomes genuinely dire. This duality is most apparent in her relationship with her boyfriend, Scorpio, another Golden Zodiac spirit; around him, she becomes kind, loving, and completely docile, threatening Lucy with violence if her usual foul mood is ever revealed to him.

In the narrative, Aquarius serves as one of Lucy's most powerful and frequently summoned spirits, often brought forth in battles that involve water to maximize her effectiveness. She is considered by Lucy to be perhaps the strongest spirit in her arsenal, possessing immense water-based magical power. Her signature ability is Water Magic, which she channels through a magical urn she carries. Using this urn, she can create torrential waves, high-pressure water pillars, and blast opponents with forceful bubbles. As a celestial being, she can also levitate and turn her body into water to avoid physical attacks. Despite her power, her cooperation is never guaranteed, and she is known to force her own gate closed and return to the Celestial Spirit World if she is displeased or has a prior engagement, such as a date with Scorpio.

The most pivotal moment in Aquarius's role within the story occurs during the battle against the Tartarus guild. Faced with overwhelming demonic enemies that her power cannot defeat, Aquarius makes a heartbreaking sacrifice. She instructs Lucy to break her golden key, an act that would sever their contract forever, in order to summon the Celestial Spirit King, a being powerful enough to turn the tide of battle. Lucy follows through with this desperate plan, shattering the key and losing Aquarius as her spirit. This event serves as a major catalyst for Lucy's development, forcing her to grow as a mage and master Star Dress Magic, which allows her to wield the powers of her spirits herself.

The story of Aquarius does not end with her key's destruction. A new key for Aquarius is later revealed to exist somewhere in the world, and the spirit briefly returns in a special space called the Memory of the Stars to share crucial information with Lucy about the past. Ultimately, through complex circumstances involving the rewriting of celestial laws, Lucy is reunited with Aquarius, restoring their bond permanently. This reunion closes the arc of loss and sacrifice, reaffirming that the deep friendship between the mage and her first spirit could not be broken by even the most desperate act.