Celestial Spirit

Celestial Spirits are magical beings from their own separate universe, certain mages can call these through the use of rare keys. Celestial Spirits each have their own separate personalities (Aquarius is extremely moody, whereas Taurus is perverted) and are ruled by the Celestial Spirit King

Appearance
While the appearance of the spirits vary greatly from one to another, each one has their own look based on the type of spirit. A few celestial spirits will even change their appearance to suit their master's preferences.

The Celestial Spirit Contract
When spirit mages receive a key they have to set up a contract with the respective spirit and 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 take extremely seriously. Stellar spirits are called from another dimension.

Abilities
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). Though Celestial Spirits aren't known for their combat abilities, a few Gold Key Spirits can actually fight. Spirits such as Taurus, Leo and Sagittarius are mostly used for combat.

Celestial Spirit King
The Celestial Spirit King who rules all of the spirit world and enforces the rules to make sure both humans and spirits abide to their contracts. It is unknown whether he hands mages his key so he can be summoned, though stellar spirit mages can summon him when they don't like the contract rules and can ask him to change them.

Zodiac Spirits
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.

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. In fact, whenever Aquarius is summoned, she always has a nasty attitude with Lucy more often then not making the situation worse then better. She has even threatened Lucy a few times for a couple of reasons (i.e.: Losing the key briefly, being summon in the middle of a water battle etc). Lucy normally only uses her when there are no other options left available. Aquarius is able to stand on land. In the past she has mentioned a boyfriend who recently was revealed to be Scorpio. She is voiced by Eri Kitamura.
 * Summoner: Lucy Heartfilia

Taurus
"The 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. Taurus has also shown to be very perverted commenting on Lucy's boobs almost every time he has been summoned (or every). Taurus' was the first summoning spirit that Lucy could force close which sends Taurus back from where he came from.
 * Summoner: Lucy Heartfilia

Cancer
"The Crab" Cancer has a weird crab-claw hairstyle with six crab legs coming out of his back, he also has black sunglasses. Cancer's seemed weapon seems to be two large pair of scissors which he can also use to cut hair, Lucy uses Cancer to cut her hair or make a new hair style for her. Happy first thought he would end Cancer's sentences with kani (crab) however Cancer actually says ebi (shrimp) at the end of his sentences, this shocked and displeased Happy.
 * Summoner: Lucy Heartfilia

Virgo
"The Virgin" Virgo has a normal black and white maid outfit, with chained shacles around her wrist. She is extremely obedient and loyal to whoever her master may be; she will even change her appearance to suit her masters wishes and says "will you punish me" after she is told to do something. When Virgo was with the Duke of Everlue, she was of an extremely tall stature and overweight with a brutish face; Natsu referred to her as "Gorilla". Now that she is under Lucy's care, she has a more typical appearance of beauty with a petite body. She enjoys calling Lucy "princess". Like her former master, Everlue, she can use diving magic, which allows her to dig into the ground or other solid substances as though they were water. It is unknown whether this is her own original magic, or if she learned it from Everlue.
 * Summoner: Lucy Heartfilia, Duke Everlue (formerly)

Sagittarius
"The Centaur" Sagittarius is a man wearing a horse costume much to Lucy's surprise. Sagittarius seems to say "hello hello" before anything he says. Sagittarius' weapon of choice is a bow and arrow, he was able to easily crush Bixlow's toys with his arrows, going right through the dolls without stopping.
 * 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 Heartphilia, 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. Karen, a stellar spirit mage, used Aries to do many horrible things for her such as used her as a shield (because apparently she has no fighting abilities) and used her to lead away all of her boyfriends. It is revealed that Angel murdered Karen and took Aries' key, although Angel doesn't seem to treat her any better, seen when Angel shoots through Aries to hit Loke during Angel's battle with Lucy.
 * Summoner: Angel, Karen Lilica (formerly)

Scorpio
"The Scorpion" Scorpio is a ruff man with a large gun. 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.
 * Summoner: Angel

Gemini
Two little spirit twins named "Gemi" and "Mini" that can transform into anyone perfectly, When they transform, they can copy a persons abilities, and even read their thoughts and memories. When Gemini was ordered to kill Lucy, they recounted Lucy's memories of her love for the celestial spirits. Gemini then broke down in tears, unable to kill Lucy.
 * Summoner: Angel414px-Gemini.jpg

Silver Keys

Crux "The Southern Cross" Crux a human like cross, with a moustache and cross shaped nose. It is unknown if Crux can fight, however Crux has the ability to search for information on Celestial Spirits and their owners by sleeping.
 * 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

Nikola
"The Canis Minor" Nikola (or Plue, which Lucy named) is a cute, yet small creature. Nikola hardly takes up magic power when summoned stated by Lucy. He has no obvious combat abilities and seems to be merely a comical character. The character is pretty much the same from Hiro's previous work Groove Adventure Rave a.k.a Rave Master.
 * Summoner: Lucy Heartfilia

Caelum

 * Summoner: Angel

The modern translation of "Caelum" is usually given as "chisel," which is not quite accurate. If "chisel" were meant, then the constellation should be called "Scalprum." The Chisel has different forms in takes, but none of them have any official names. These names given are just what I think they might be called.
 * "The Chisel" is the modern translation of the constellation Caelum, but apparently that's not quite accurate. The word is ambiguous, in Latin caelum means both "the heavens" and "birin," which is an instrument used for engraving on copper and fine metals. It is this instrument that Lacaille had in mind when he named the constellation.

1. The Cannon Form: The first form to be used and a therefore a good candidate for it's default form. In this form it appears as high tech cannon that can launch powerful blasts of energy. It can be assumed that Caelum has control over the path in which it fires its beam as it curves it around Lucy to avoid harming her. It can be assumed that Caelm has emotions, since there could be no other reason it would choose to curve the beam. 2. The Flight Form: The second form to be used. It is a another good candidate for Caelum's Default form because in this form it appears as a simple winged sphere which seems to make it the basis for the other forms. This form has yet to be seen using any attacks and it probably has none. Though it's likely this is the only form shown that can move considering it's the only one that looks like it could, but it hasn't been seen doing this yet, so there is no proof of this. 3. The Sword Form: This form has the "Chisel" part of the sphere enlarged so much it looks like the Chisel is the blade of a sword with the Sphere part being it's hilt. This form is in fact the first evidence of some stellar spirits having no ability to resist their masters and it seems Caelum even lacks a personality or speech abilities which makes it very different from other stellar spirits shown so far. The proof while it's wielder Gemini stopped attacking Lucy when it realized Lucy's love of her kind. Caelum still seemed to be under Angel's control as if it couldn't feel Lucy's emotions or if it could didn't feel any emotions of its own; this is, however, assuming that it curved the path on purpose.

Trivia

 * Lucy has said there is a "Draco" Celestial Spirit after Natsu stated he wanted a dragon one.
 * An early version of Taurus can be viewed in Hiro Mashima's earlier work Monster Soul in ch2 pg49
 * "The Chisel" is the modern translation of the constellation Caelum, but apparently that's not quite accurate.