A brief run-down of the Celestial subgenres, just so you know what else exists in the world:

The Spirit: A spirit of a place is the embodiment and personification of the harmony and purpose of that place. For example, a Spirit of the Forest might guide the animals within to find food, or help the flow of water to create pools for the animals to drink from. If the forest is attacked (say, by a logging corporation that doesn’t practice sustainable replanting, for instance), the Spirit of the Forest might guide the animals or even the weather and other forces of nature to drive out the invaders. The spirit seeks to fulfill the highest purpose of its domain, and will usually seek harmony and peace to that end. Sometimes the Spirit may manifest in a person, an animal, an apparition, or just an emotional “feeling” to communicate. Examples include Aewa, the planetary spirit from Avatar, Shai Hulud from Dune, Moia the living ship from Farscape. Off-beat example: The Dude.

The Shaman: A Shaman is in tune with the emotional energies of everyone around him or her. He or she can communicate with the unseen world, can sense emotional energy acutely, and can wield emotional energy to help others fine tune their state of being. He or she is adept in wielding metaphors (art, storytelling, song, etc) to adjust emotional states in people, and lead them to greater insights about themselves. The Shaman possesses an extraordinary sense of his or her own emotions, and is very difficult to perturb. He or she is content to do his or her job and exist unseen in the background, a plot point in a larger story rather than a character in his own. Examples include Uncle Iroh, Albus Dumbledore, Charlotte the spider.

The Questor: Where there is the threat of disorder, the Questor seeks to restore order. He might be a knight, an adventurer, a traveling justicar, a monster hunter, a traveling scholar, a hapless hero, or anything else that seeks to impose order by force of his own will or agility of his own wit. A Questor is loathe to shy away from a challenge, especially when that challenge represents the chaos he is working against. The Questor will face any danger with only the quality of his own character if need be, and will stop to help any in his path who may need it. Examples include McGuyver, Sir Gawain, Sam and Dean Winchester.

The Angel: The angels see their mission as broader than mere protection. They seek to address the broader issues that make that protection necessary in the first place. While a Guardian Kaiju may stand in the way of bullies for a tormented youth and frighten away the youth’s attackers, the Angel will want to know what is wrong in the child’s home life that made him unable to protect himself to begin with. The angel concerns himself with the holistic functioning of the order of a particular domain: is the economy doing well? Are the police doing their jobs with efficiency and courtesy? Is a particular concern being systemically ignored? Examples include Sam from Quantum Leap, Captain Jean Luc Picard, Constable Benton Fraser.

The Sacred Princess: To rule a domain is an honor, a privilege, and a responsibility. To be the authority of any domain, the Sacred Princess must embody all the best intentions and aspects of her dominion. Her behavior sets the example and standard for all. She must have an understanding and rapport with all her subjects and her inner circle, and balance the needs of her entire dominion as carefully and as equitably as possible. Examples include Zelda, Mr. Mom, Fluttershy.

The Adept: The adept is a self-disciplined ascetic who has mastered his own body and mind, and can make both adapt in sometimes fantastical ways. He has usually studied eastern philosophies and religions, probably one or more martial arts, and is at peace with his own existence. An adept might have multiple alternative forms, or might just content himself with animistic stances and movements. The ultimate goal of the Adept is to make himself or herself better in every way possible. Examples include Bruce Lee, Severus Snape, El Zorro.

The Chosen One: The Chosen One is a special member of the community, bearer of that communities hopes and dreams, and in some cases their fears. The Chosen One’s job is to be an emotional focus for that community, and solve some specific problem for them. The Chosen One is typically denied a normal childhood, instead having to train intensely to fulfill his or her destiny, but as a result, will have been bestowed certain abilities by the focus of that emotional energy. Examples include John Connor, Avatar Aang, Chuck Bartowski.

The Fae: The Fae are ethereal creatures formed by emotional energy, and possessed of heightened consciousness and complex society. The Fae are proud, separate from normal Humanity, and seek to govern the affairs of the emotional energy within their domain without interference from others. They have strict codes of etiquette and communication with other creatures, and do not brook insult or usurpation of their authority. They exercise their power through boons, curses, and geasa. They do not assist Humans without the fulfillment of some price, even if it only be a favor to collect later. Examples include The Inhumans, The Devil, Nina Sharp and the company Massive Dynamic from Fringe.
Note that the Fae are not limited to Celtic mythology. Every culture had similar creatures: tanuki, djinn, talking animals, daevas, cherubim, cthonian gods, vanir, spirit-walkers, and so on.

The Council of Wizards: The Council of wizards has never met in person. They only meet aetherialy, and usually only through metaphor. They work their magic by metaphysical agreement, and can spend long hours, days, weeks, even years carefully constructing the statement to be agreed upon, or the question to be asked. They are acutely attuned to the flow of all energy in the world, and always know when there are fluctuations in those energies. They always have their hand in the large scale events of the world, but surprisingly have very little interest in details that others might deem to be of utmost importance. They might not openly address the genocide in whatever third world country just made the news, but they might show focused interest in a particular court case in some mid-sized town in North Dakota. One day they might dismiss the words of a politician as “Just politics,” but then a week later parse the careless words of that same politician’s top aide in an interview as if filtering it for a coded message. The goal of the Council of Wizards is to maintain the good functionality of the system of the world, and they use their collective magic to address the big questions. Examples may include The Maiar of Middle Earth, The Watcher Council from Buffy the Vampire Slayer, SHIELD.

Celestials as player characters:

Well, I don’t object to it, but I’d really have to write another source book. That’s not to say you and your GM can’t agree on some schema for converting a Kaiju character template into a Celestial, but you and the GM will have to wing that, since I haven’t written it up yet. If you have any good ideas, pass them along to me.
But the larger answer to that question is, since the eruptions, both Kaiju AND Celestials have returned to Earth in more concrete and literal ways than the metaphors they existed as before. There is magic again, and those more things in Heaven and Earth are now being dreamt of in all philosophies with frightening certainty. Walking by faith has now become walking by sight. Both the Kaiju and Celestials agree on one thing: the state of the world is pretty crappy right now, and we need to get to work.

To the GM:

This is your chance to really cut loose and play. Since I gave only vague descriptions of the Celestial types, feel free to make stuff up. Throw your players a curve-ball, make them adapt on the fly. The Celestials are your set-pieces to have fun with. The less your players know about them going in, the more dynamic the game will be.