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Crowley was originally the angel Kokabiel and here's why:
Just looking at this definition of Kokabiel, we know two things: 1.) that they were the angel in charge of the stars, and 2.) the ended up falling. This screams Crowley, as in the first episode of season two we are shown Crowley in his angel form literally creating the stars. He also has a map of the stars in his hand, including all the constellations.
When the galaxy comes alive, Crowley is ecstatic! But when Aziraphale asks if he had done it all himself, he says "Ah, well, I mean, more or less. I wasn't... I wasn't, um... I wasn't the original concept designer, but I worked very closely with the upstairs on it," The "upstairs" in question being God.
Continuing on this theory, Crowley then explains that what he's created is a "star factory" and then launches into teaching Aziraphale about the creation of stars and constellations--much like Kokabeil was known for.
Once Crowley is let on that the "upstairs" is going to shut everything down in about 6,000 years, Crowley is mostly upset that his stars would die. He doesn't know earth by it's name, as he would most likely never have concerned himself with anything other than stars, and when he's told that his stars are only there for humans to just look at, he gets upset and ultimately starts questioning God--a slippery slope after all, because how much trouble can you get in for asking a few questions?
Well, a lot, apparently. Crowley becomes a fallen angel, like Kokabiel, and as we see towards the finale, he still has enough power to open up Gabriel's classified documents--something only a very powerful angel could do. This would align with Crowley being Kokabiel, since Kokabiel was the angel behind creating the universe and all it's stary components.
Not to mention that Crowley has dropped hints about his true name throughout the show, constantly bringing up that they could runaway together to somewhere in the universe both heaven and hell can't find them, or to alpha centuari--a star system in the universe. This hints that Crowley knows places in the universe that others do not (since he created it), and his desire to run away into the stars is just setting up reveal it is because they are of his own creation as an angel.