Celestial Spirit

For the related Magic, see Celestial Spirit Magic.

Celestial Spirits (星霊 Seirei) are Magical beings from their own separate universe, the Celestial Spirit World. Some wizards, called Celestial Spirit Mages due to their specialization in Celestial Spirit Magic, are able to summon Spirits by opening their Gates through the use of Celestial Spirit Gate Keys. These Keys are separated into two classes: the more common Silver Keys and the extremely rare Gold Keys. Keys for Celestial Spirits are counted in Units (collectively without regard of Gold or Silver), and a Celestial Spirit Mage is noted by how many Units they have obtained. All Celestial Spirits are each named after a different astronomical constellation. Out of the 12 Gold Keys, 10 are owned by Lucy Heartfilia and the remaining two are owned by Yukino Aguria, who also holds the key to the "13th Gate" of the Zodiac.

Contracts and Rules
When a Celestial Spirit Mage receives a Key and opens its Celestial Gate for the first time, they have to set up a contract with the respective Spirit. This contract consists of asking the Spirit which days it can be summoned by the Mage. This simple agreement, much to the shock of Natsu and Happy, forms a dedicated and serious bond between the Spirit and summoner. However, under certain circumstances, i.e. during battle, a contract can be asked to be completed later.

Contracts can be broken if the Mage is arrested, releases the Spirit by him/herself, or dies. When a Celestial Spirit Mage summons a Spirit, it appears directly next to them, as that's where the Gates to the Spirit World have been opened. It's impossible to summon Spirits elsewhere. The Spirits must also abide by certain rules enforced by the Celestial Spirit King. The only rule that has been introduced in Fairy Tail so far is that a Spirit may not directly or indirectly kill its summoner under any circumstances. When the Gates are closed, it is required from both the summoner and the Spirit to agree upon the Gate's closure. However, Spirits can also be "forced closed" in battle, which means they can be forced back into the Celestial World by the summoner's will, though not every Celestial Spirit Mage is capable of doing that.

Celestial Spirits also have different categories of sorts, which fall under the basic summoning conditions from their owner. For example, Loke summons himself most of the time without Lucy doing so herself, Virgo also has summoned herself once, as have Gemini and Horologium and if trust between the Celestial Spirit and the owner is not an issue, contracts can be temporarily dissolved. It also seems that if the Celestial Spirit is strong enough, they can use their own Magic to stay in the Human World, when or if the contract is not in action. The strength of the Celestial Spirits is affected by the Magical strength of their summoner as Lucy once said that her spirits could become even stronger than Natsu if she trained more.

Trivia

 * Nakajima from Rave Master made a brief cameo in episode 18, when Lucy talks about the Celestial Spirit Sagittarius and Natsu imagines an octopus with a Nakajima flower head.
 * Originally, summoning Celestial Spirits were supposed to be by cards. However, this idea was eventually scrapped when the author thought that it was too similar to another manga work.
 * During her time in the Tower of Heaven, Lucy managed to summon both Cancer and Aquarius during the same day, while from their contract, Cancer can only be summoned on Tuesday, Thursday, Saturday, and Sunday, while Aquarius can only be summoned on Wednesday. This question was later answered in the Q&A of Volume 16 that the contract is actually constantly changing, and the stronger and trustworthy the bond between the Mage and the Celestial Spirit, the summoner can increase the amount of days that a particular Spirit can be summoned.
 * The Golden keys appear to be more stylized in the anime, appearing with handles and teeth to the theme of the summoned Spirit, while in the manga they appear as ordinary keys with the Constellation's symbol engraved on the top. Recent chapters in the manga depict them in their stylized forms, as seen when Lucy summoned Taurus and Scorpio simultaneously or when Yukino offered Lucy her two keys.
 * A large number of unnamed Celestial Spirits appear during the party Lucy's spirits hold for her and her friends in the Celestial Spirit World; most of them reflect constellations amongst the 88 Modern Constellations.
 * It is stated that if all of the 12 Ecliptic Zodiac come together, they will open "the gateway that will change the world".