Celestial Spirits (星霊 Seirei) are magical beings from their own separate universe, the Celestial Spirit World. Mages who possess these spirits, in the form of keys, are referred to as Celestial Spirit Mages. These keys, when used, open a doorway into the Celestial Spirit World, and allows the user to summon the spirit of its respective key. Only one key exists per spirit, therefore, once a key is in someone's possession, only they can summon that Celestial Spirit until the key is broken, or is in another's possession.
Celestial Spirits are named after different things that deal within the realm of astronomy.
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, 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 gets arrested, releases the spirit by him/her self, 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 had 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, that 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/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.
Types of Keys
Diamond Keys: Diamond Keys are the second least common of all keys. They are only handed down to extremely trustworthy Celestial Spirit Mages, given to them by the Celestial Spirit King himself. Only one key can be held per Celestial Spirit Mage. These keys cannot be broken, and when used, it closes the gate of all other active Celestial Spirits summoned by the user. When the Celestial Spirit of a Diamond Key is being used, no other Celestial Spirits may be summoned until its gate is closed. They are considered to be stronger than Golden Keys, as all of the Diamond Keys open the gate of a Celestial Council member, where there are only 8 members. Diamond Key Celestial Spirits are named after planets.
Golden Keys: Golden Keys are very uncommon keys. They are able to found within the Human World, whether they are in the possession of another Celestial Spirit Mage, or none at all. It is said that when all 12 Golden Keys are together, they are able to open a gate that could change the world. Unlike Diamond Keys, they are able to be used in multiple supply, but they do require more magical power to keep open than Silver Keys. Golden Celestial Spirits are named after constellations.
Silver Keys: Silver Keys are the most common type of keys. They contain spirits of more variety than the other keys, and some keys can be seen sold within towns and the like. While they are not as strong as Diamond or Golden Keys, the Celestial Spirits that are summoned do not require much magical power, making this the most easiest type of spirit to summon within the keys. Silver Key Celestial Spirits are named after stars.
Obsidian Keys: Obsidian Keys are the most uncommon of all the keys. They are considered to be cursed keys, where they will continue to sap away the magical power of their user until it's completely depleted. Obsidian Key Celestial Spirits cannot have their gates closed forcefully except when a mage uses a Diamond Key. These keys cannot be found within the Human World, as they can only be created by forming a contract with a spirit of this key within the Celestial Spirit World. These keys cannot be broken, and will disappear when they are used, having the mage renew its contract with the spirit. The spirits summoned by these keys are considered to be the strongest Celestial Spirits, only below the Celestial King himself. Obsidian Key Celestial Spirits are named after galaxies.
Diamond Key Spirits
Mercury, The Small (小さな嵐 Chīsana arashi)
Venus, The Quadrupedal Whirlwind (四輪旋風, Yonrin Senpū)
Venus is a calm and collected Celestial Spirit that is apart of the Celestial Council within the Celestial Spirit World. She only relies on her magical capabilities, as well as her extensive knowledge, for battles, since she isn't physically strong. She boasts a large amount of magical power, putting most other Celestial Spirits to shame. Her primary form of magic is manipulation of her hair. However, she has been shown to be very capable of magics such as Wind Magic and Fire Magic. She has also shown to being able to know the location of every other Celestial Spirit. Her duties within the council consist of keeping track of the whereabouts of the Celestial Spirits at all times.
Earth, The Good Mother ()
Earth is an eccentric and loud Celestial Spirit that is apart of the Celestial Council within the Celestial Spirit World. Her magic mainly relies on restoring things to a more proper state. She has no combat abilities, only capable of acting as a support within battles. Despite having no sort of combat capabilities, she is a very durable Celestial Spirit, being more than capable of taking hits without flinching in the slightest. She sometimes claims that she didn't feel anyone harm her. Her duties within the Council consist of her keeping everything within the Celestial Spirit World healthy, primarily the Celestial Spirits.
Mars, The Sands of Time ()
Mars is a quiet Celestial Spirit that is apart of the Celestial Council within the Celestial Spirit World. He relies on Earth Magic its variations, mainly Sand, and Time Magic. He is known for infusing his sand with his Time Magic, and those caught within his sand slow down, giving him his title. Due to his body being made out of sand, because of his magic, he is able to stretch his limbs, or create more, and attack his opponents from a distance. Outside of battles, he can be seen roaming around, taking the views. His duties within the council consist of maintaining the Celestial Spirit world's kind atmosphere, resolving conflicts between spirits.
Jupiter, The Shadow Storm ()
Saturn, The Crater ()
Uranus, The Pusher ()
Neptune, The Ice Sculptor ()
Neptune is an arrogant and wise Celestial Spirit that is apart of the Celestial Council within the Celestial Spirit World. His primary form of fighting revolves around the usage of his tongue, sticking it to his opponents and freezing them. When not trying to freeze them, he he will often reprimand his opponent(s) for attempting to fight him due to their ignorance. He is a very proficient user of Ice Magic, capable of completely freezing areas without too much of a hassle. Whenever he is given an owner, he often disobeys their orders, believing that only his actions are acceptable. Due to this, very rarely is his key given out. He is the head of the Celestial Council, only answering to the Celestial Spirit King.
Gemini, The Fraternal Twins (友人の双子 Yūjin no futago)
Gemini are twin Celestial Spirits that tend to joke around a lot, especially within the midst of battles. While they themselves hold no physical capabilities, they are very proficient in the usage of magic, especially Copy Magic. Aside from Copy Magic, they are capable of using Paper Magic. Despite their virtually opposite personalities, they seem to get along with Polaris very well. They are one of the most used Celestial Spirits within Polaris' arsenal, claiming that they are not replaceable. He has also used their Copy Magic to train to better improve himself, and them.
Hydra is a two headed Celestial Spirit that seems to be very protective of those that dwell within it. He is considered to be one of the largest Celestial Spirits, being about 5x larger than an average human. Due to his size, he boasts a high amount of endurance to enemy attacks. He holds some physical strength, but is very slow in combat situations. He mostly relies on Earth Magic within his battles, but he is capable of using Light Magic to an extent, where its source comes from the lanterns on his body. Polaris mainly uses Hydra when he is out on his own and needs a place to sleep in. Hydra is capable of digging underground and faking the appearance of a real tree.
Libra is a blind-folded Celestial Spirit that has a calm and collected demeanor. She is a very strong Celestial Spirit that primarily relies on Water Magic and Gravity Magic within her battles, as well as the usage of the sword she wields in her left hand. She has shown to possess the power to weigh the chances of something with her scales. Her predictions have yet to be incorrect, as every time she has been asked to weigh something, it has proven to be the truth. Polaris mainly uses this ability to weigh the odds of a battle he is unsure of. Despite being blind-folded, she is very capable of telling where everything is, having a very acute sense.