Celestial Spirit

Celestial Spirits (星霊 Seirei) are magical beings from their own separate universe, the Celestial Spirit plane. Mages, called Celestial Spirit mages, are able to summon spirits by opening their gates through the use of keys. These keys are separated into two classes: the more common silver keys and the rarer gold keys.

All Celestial Spirits are named after a different constellation.

Contracts, rules and abilities
When Celestial Spirits mages receive a key and open the gate for the first time they have to set up a contract with the respective spirit which consists of asking the spirit the days in which they can be summoned. This rather simple contract (much to the shock of Natsu and Happy) forms a bond between the Spirit and summoner that is taken extremely seriously. However, under exceptional circumstances, such as during the heat of battle, a contract can be asked to be completed later. Contracts can be broken if the mage is arrested or dies.

The spirits must abide by certain rules, enforced but not set by the Celestial Spirit King. So far the only known rule is that a spirit may not under any circumstances, whether directly or indirectly, kill their summoner. Spirits also can be "forced closed" in battle which in essence is sending the spirits back to the Celestial World before the summoners magic has been depleted.

All spirits have a different ability that makes them completely unique (Aquarius can control water, Lyra can sing and Crux knows a lot about about other spirits). Most silver keys are not known for their combat abilities, however a few can fight, for example, Caelum. So far every seen gold key, on the other hand, can fight in some shape or form with some more combat potent than others for example Lucy states when summoning Aquarius that she is the most powerful spirit she has in the Oración Seis arc, but she can only be used in areas with water. The length a Celestial Spirit can be summoned depends on the amount of magic the user possesses and whether the key is golden or silver. According to Angel, Celestial Spirits are immortal. But this is only partly true, under normal circumstances they are immortal, but extended periods of time in the human world can be painful to the spirit, constantly losing magical energy they are made of, eventually after losing all their magic, they die permanently. it takes quite a long time for this to happen depending on the spirt's power, as Leo was out for years before he became close to death.

Celestial world
All Celestial Spirits spend their time in the Celestial Spirit world when not called through by their summoners. Being in the world seems to have some sort of rejuvenating effect on the spirits, restoring their health after being in the human world which, over long periods of time, can potentially kill the spirit. The reverse is also possible: humans are unable to breathe in their world so after a period of time, albeit a much shorter one, they would die. Such an occurrence (a human entering the Celestial world) is a serious breach of contract.

But even so, by accident Natsu entered the Celestial Spirit World. He was trying to catch one of the 12 Zodiacs, the Maiden, and by mistake he got pulled with her to the spirit world, when the Maidens gate closed.

Celestial Spirit King
The Celestial Spirit King (星霊王 Seirei-ō) who rules all of the spirit world and enforces the rules among all the spirits. He is the most powerful of all the spirits although it is unknown if he gives out his key to humans or even if he has one.

Gold Keys
Spirit mages have the potential to obtain extremely rare Gold Keys, which open the gates of the "Ecliptic Zodiac". According to Lucy, while there are large number of silver keys, there are only 12 gold keys. Each gold key has the Zodiac symbol of the one it is used to summon on it. The only remaining Ecliptic Zodiac Spirits left to be seen are Libra, Pisces, and Capricorn.

Aquarius
"The Water Bearer" Aquarius is in the form of a mermaid carrying a jug of water. Aquarius is one of Lucy's most powerful spirits, but also the hardest to control. When she attacks Lucy and her opponent are carried away with water. She also dates the Celestial Spirit Scorpio.
 * Summoner: Lucy Heartfilia

Taurus
"The Golden Bull" Taurus is a minotaur with black and white splotched skin, with a nose ring and black speedo's with a belt. Taurus' preferred weapon is a giant two-bladed axe with Taurus' strength it becomes a deadly weapon. He is also quite a pervert.
 * Summoner: Lucy Heartfilia

Cancer
"The Giant Crab" Cancer has a weird crab-claw hairstyle with six crab legs coming out of his back, he also has black sunglasses. He wields a pair of sissors that he expertly uses for both melee attacks and haircuts. Happy thought he would say "-kani" (Japanese for "crab") at the end of the sentence, while in fact he says "-ebi" (Japanese for "shrimp"). Happy was shocked and even asked Lucy to dismiss Cancer.
 * Summoner: Lucy Heartfilia

Virgo
"The Maiden" Virgo has a normal black and white maid outfit, with chained shacles around her wrist. She has the ability to dig holes underground. She is extremely obedient and loyal to whoever her master may be; she will even change her appearance to her master's liking. She also asks her master if they will punish her after she has helped them, even if she has done a great job, indicating she may enjoy it.
 * Summoner: Lucy Heartfilia, Duke Everlue (formerly)

Sagittarius
"The Archer" Sagittarius is a man wearing a horse costume much to Lucy's surprise with a great skill in archery, as setting a machine on flame with arrows is child's play to him. He tends to say "hello hello" and the beginning or the end of the sentence.
 * Summoner: Lucy Heartfilia

Leo
"The Lion" Leo has his hair in the form of a lion's mane. Leo the Lion is later found to be Loke, and joined Lucy after she helped him return to the spirit world. Loke seems to be able to sense when Lucy is in danger and come to her rescue even without the keys or being called. Leo uses Regulus, a form of light magic, Loke is also able to use Regulus, but because of his weakened state, he could only channel it through his magic quartz ring.
 * Summoner: Lucy Heartfilia, Karen Lilica (formerly)

Aries
"The Ram" Aries is a timid female. She has horns at the top of her head much like a ram. She has white and fluffy clothes, like a sheep. Her magic deals with using cloud/wool like substance for combat. She is good friends with Loke, who risked his life trying to free her from her former summoner Karen's treatment.
 * Summoner: Lucy Heartfilia, Angel (formerly), Karen Lilica (formerly)

Scorpio
"The Scorpion" Scorpio is a rough man with a large gun for a tail. Not much is known about his abilities as of yet but he is Aquarius's boyfriend. Apparently the two have a happy relationship with each other. Scorpion strangely refers himself as plural, saying "we" instead of "I" in conversation. The reason is unknown but it might be possible that he counts his tail as one person.
 * Summoner:Lucy Heartfilia, Angel (formerly)

Gemini
"The Twins' Two little spirit twins named "Gemi" and "Mini" that can transform into anyone perfectly and replicate both their thoughts and magic. They were formerly Angel's most used spirit but left her when they recognized Lucy's love for Celestial Spirits.
 * Summoner: Lucy Heartfilia, Angel (formerly)

Crux
"The Southern Cross" Crux a human like cross, with a moustache and cross shaped nose. Crux has large information about all Celestial Spirits and their masters, either in the presence or the past. He looks like sleeping when searching for the information, which confuse other people beside Lucy (like Happy) with whether he is actually sleeping or otherwise.
 * Summoner: Lucy Heartfilia

Horologium
"The Clock" Horologium is a large grandfather clock. Horologium has combat and non-combat uses. Examples of non-combat uses are shelter from snow and other climates or transport, while combat examples of Horologium would be defense hardening by hiding inside of him. Lucy always uses Horologium for shelter and when she talks, Horologium would repeat her words then end with "she says" after each sentence.
 * Summoner: Lucy Heartfilia

Lyra
"The Lyre" Lyra wears a bonnet and dress. She also has wings on her back, she has the ability to sing beautiful songs, she mostly sings about emotions, but she does take requests. Lucy mostly uses her for musical theater's or competitions.
 * Summoner: Lucy Heartfilia

Nikora
"The Canis Minor" Nikora (or Plue, which Lucy named) is a cute, yet small creature. Nikora hardly takes up magic power when summoned stated by Lucy.
 * Summoner: Lucy Heartfilia

Caelum
"The Chisel", it appears as more of an item than a being which takes three different forms to attack. So far this is the only Silver Key shown to be used in combat. It is unknown what happened to it once Angel was defeated, but has most likely ceased its contract like the others, yet did not join Lucy.
 * Summoner: Angel (formerly)

Trivia

 * Nakajima From Rave Master made a brief cameo when Lucy talks about the Celestial Spirit Sagittarius and Natsu imagines a octopus with a Nakajima flower head.
 * In the horoscope their are 4 elements Fire, Water, Air, and Earth. Out of all the ecliptic spirits revealed so far their have been one female spirit for 3 of the elements, Aries for Fire, Aquarius for Air, and Virgo for Earth. If the author continues this pattern their should be at least one more female out of the three remaining spirits, likely Pisces, since Cancer and Scorpio are also in the water group but have already been introduced.
 * The author may also put one female spirit, a weapon using spirit, and a magic using spirit for each element as seen with Aries, Sagittarius, and Leo who are all in the fire group. If this is the case then in the Earth group they need a magic user, in the Air group they need a weapon user, and for the water group they need a female spirit.
 * Originally, summoning Celestial Spirits were suppose to be by cards. However this idea was eventually scrapped when the author thought that it was too similar to another manga work.
 * So far, Caelum is the only Celestial Spirits from both Gold and Silver Keys that is not belonging to Lucy that have appear in the series.
 * The "88 signs" from Lucy's Uranometria incantation refers to the 88 modern constellations. Every celestial spirit (gold and silver) used in the series thus far derives its name from that list and it is therefore possible that the spell calls upon the power of all the celestial spirits at once.
 * The Spirits are named after astronomy constellations
 * During the Tower of Paradise arc, Lucy managed to summon both Cancer and Aquarius during the same day, which 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.
 * So far of the Zodiac keys, Lucy actually owns 10 out of 12 keys.