Lucy Heartfilia is one of Fairy Tail's main characters, and its in-series narrator. While the magic of her colleagues is fairly straightforward fare for a shonen anime, Lucy's Celestial Spirit magic is a bit more mysterious, and has fewer analogues in other series. '
As Fairy Tail's anime is set to return for an adaptation of the sequel manga, 100 Years Quest, now's as good a time as ever to examine how Lucy's magic works.
What are Celestial Spirits?
The Celestial Spirits are powerful magical beings who inhabit their own separate world, the Celestial Spirit World. Each Celestial Spirit (except for the extremely powerful Celestial Spirit King) is named and modeled after a real constellation. Celestial Spirit Mages like Lucy use their magic power to summon these spirits to aid them in battle or perform other useful functions. This is usually done through the power of Gate Keys, which are each attached to an individual spirit. Lucy has been shown to summon her spirits without gate keys in Elentear, a world with excess magical power, but is otherwise forced to use the keys as a medium.
Gate Keys are divided into three known categories. Silver Keys represent lesser constellations and are relatively common. The spirits attached to them are weaker, and some are only used outside of battle. The total number of Silver Keys is unknown, but they are greater in number than the much rarer Gold Keys. There are twelve Gold Keys in all, each representing a different constellation in the zodiac. Each of the Zodiac spirits carry power parallel to those of experienced wizards, and are entirely sentient, with their own unique personalities.
Lucy currently possesses 9 Gold Keys, belonging to Aries, Taurus, Gemini, Cancer, Leo, Virgo, Scorpio, Sagittarius, and Capricorn. She also owns the Silver Keys for Horologium, Crux, Lyra, Pyxis, and Nikora. Yukino Agria, a Celestial Spirit Mage from the Sabretooth Guild, owns the Gold Keys for Libra and Pisces, the Silver Keys for Caelum, Polaris, and Deneb, and a mysterious Black Key which summons the serpentine spirit Ophiucus. Aquarius's key once belonged to Lucy, but during Fairy Tail's battle against the Dark Guild Tartaros she was persuaded by Aquarius to break it in order to summon the Celestial Spirit King. Although Lucy initially thought Aquarius had died, it was later revealed her key has regenerated in an unknown place. Lucy is currently looking for the key again.
How Summoning Spirits Works
Celestial Spirit Mages are able to summon their spirits after entering contract with them. The contracts determine the specific days a spirit can be summoned and are only rendered null and void if a Celestial Spirit Mage dies, is arrested, or voluntarily releases the spirit from their servitude. Spirits are also forbidden from killing their summoners, but this doesn't prevent them from lashing out at them physically. In Lucy and Yukino's cases, and under ideal circumstances, Celestial Spirits and their mages form strong bonds and work in tandem with each other, with each making the other stronger. Other Celestial Spirit Mages, however, have treated their spirits far more cruelly, essentially as slaves.
Under normal circumstances, Celestial Spirits remain in their own world unless summoned by their mage, but some like Leo and Virgo frequently summon themselves. It's implied that this doesn't drain Lucy's magic the same way as when she summons them through her keys. Similarly, while sending a spirit back typically requires mutual consent, Lucy has been able to force gates closed, and Aquarius has returned to the Celestial Spirit world without Lucy's permission several times.
Typically, Lucy only uses one Celestial Spirit at a time in battle, but summoning multiple spirits at once is possible. There also seems to be no strict upper limit on the number of spirits summoned, but doing so drains a massive amount of magic, eventually risking the mage's life. As a result, summoning multiple spirits is usually a last-resort gambit, because of the risks involved.
The rules affecting the use of Celestial Spirit Magic are among the most complex in Fairy Tail, and largely serve to prevent Lucy and Yukino's magic from being completely broken. This is due to the fact that each Celestial Spirit indirectly grants its user access to a different form of magic, making Celestial Wizards capable of a wide variety of different attacks. Hiro Mashima wisely added caveats to deal with that.