The Celestial Guardians (also known as the Guardian Angels of Order) are the protectors of Order and are among the oldest of beings in the universe who live in the highest part of the Dark Dimension. They act as policeman, but do much less than they actually could do in order to stop crime in the city. Elena describes them as selfish and cold in the book Shadow Souls.
Not much is known about these Guardians, but they are among the oldest creatures in the universe, they are as old (and possibly older) as the Old Ones, the Original Pack, the Original Phantoms and the first Demons who lived at and/or before the foundation of human civilization. They are benevolent and possess a great deal of Powers that allow them to perform a number of supernatural acts with great force.
In the Books
The Guardians don't make many appearances in the book. When Elena and her friends first arrived in the Dark Dimension to look for Stefan, they were inspected by a very beautiful woman with a striking resemblance to Elena. The woman was a Guardian and she was more interested in Elena than all others. The Guardian then let them into the city.
Later, when Elena and her friends found Stefan, they were chased by a giant bird. The Guards do their best to make sure there is as little damage as possible, attacking the bird while helping the community escape. Elena and her friends successfully escaped the jail and made their way back to Earth.
Stefan, Damon, Bonnie and Elena travel to the Dark Dimension again to find the starball of Inari, a fox goddess who was Shinichi and Misao's leader. Elena and her friends traveled to the Nether World where they found the Seven Kitsune Gates, which are Gates that lead to Seven Paradises. The Gate Keeper, Sage, only allowed them to enter one Gate. While Elena yearned to enter the Gate that leads to the Fountain of Eternal Youth and Life, they all decided on a door which led them to a moon in the Nether World with a world tree growing on the whole planet. Shinichi attacked them on a branch and ended up dying when he ultimately regretted ever coming into their lives, as the sin of regret can be fatal to kitsunes.
The protagonists then found the starball on a section of the tree, but when they tried to grab it, the tree unleashed its branches and killed Damon by staking him through the heart. Elena, heartbroken, unleashed her Wings of Destruction and burned the tree to ashes, destroying the starball and almost destroying the whole world/moon in the process before she passed out.
When she awakened, Sage informed her that their quest was not for nothing, and they started forming a plan that would get the attention of the Celestial Court: they would break the rules of the Seven Kistune Gates and take as many things to bargain with the Celestial Guardians for their help as they could carry. After they raided the gates, the Celestial Guardians arrested them and took them to the Celestial Court. The team then discovered that the car accident where Elena's parents died was a mission of the Celestial Guardians gone wrong, as they were there to kill Elena to make her a Guardian. As a result of this unintentional outcome, they were happy to help to to make up for their mistake their fault. Elena angrily wished that her life was as if Klaus had never come into their life before making the following demands: they must bring Sue Carson and Vickie Bennett back from the dead, erase all the problems caused by the Kitsune, and she wanted who ever their master is "dead". She also wanted the Guardians to make it as though she had never died and for everyone to accept Stefan as her boyfriend. The Guardians accepted this with no problem until Elena stubbornly asked for Damon's life back. The Celestial Guardians informed Elena that they could not, because they have no power over what happens to vampires' souls when they die, and assure her that they had already searched for his soul and could not find it. Elena became extremely infuriated by this and re-summoned her Wings of Destruction. The Guardians retaliated and stopped her before she could destroy them all before stripping her of all her Wings Powers, though they did let her keep the aura of her blood to use as a weapon against any Old Ones. The Celestial Guardians even change reality in the way that Elena demanded despite this reaction, and Elena awoke in her bed on Earth