Originally published October 17 2013 on Tumblr.
Castiel is still an angel. A fallen, degraced, dewinged angel, but still an angel. He is human-like (mortal, feels hunger and pain, and so on) but not human. He retains some angelic abilities which are apparently not grace-dependent. He can still tune in to “angel radio.” There are probably other things that we have not seen yet, such as possibly recognizing angels and demons. (But this may also be a grace-dependent ability, as he may or may not have recognized Hael as an angel. “You’re an angel” might simply have been the most logical conclusion, or he knew her name. He certainly didn’t react as he walks past her, as one might expect if he recognized her as an angel.) It’s an important distinction, although one that seems generally ignored in favor of Castiel’s supposed humanity.
“Human” seems to be a simplified shorthand for an ungraced angel. It’s how season 9 Castiel has been publicly presented and referred to by TPTB; most recently Carver referred to Cass as “human.” In the show, it’s also a shorthand we’ve seen Dean use before; in “Two Minutes to Midnight”, he summarizes Cass’s description of his complaints and ailments with “Human.” To Dean, an angel is a being possessed of its grace and power and attendant abilities; minus that, he equates the now-mortal being with humanity. This is not a negative necessarily; but simply his view that an angel is by definition a creature with powerful abilities that can be used in his favor or against him. Minus those powers, he sees them as the same as him—human beings. Accordingly, he sees these beings with human form and human frailties as human. It’s an understandable conclusion, but an incorrect one.
There are two major examples of degraced angels: Castiel at various points (most notably in “The End” and the end of Season Five) and Anna. In “The End”, Cass tells Dean he is “not an angel anymore” to describe his fallen state, but that may be a simplified according to Dean’s understanding. If we accept that “The End” was indeed one potential future, it is easy to suppose that at some point Alternate!Dean referred to Castiel’s loss of “mojo” as him becoming “human”, as he did in the “true” timeline when Cass showed up in the hospital in “Two Minutes to Midnight,” and Cass therefore describes himself accordingly. Alternately, if “The End” is a fabrication created by Zachariah (which is not my personal view but an interpretation favored by some) it makes sense that Castiel would speak in the same terms as Dean, equating fallen with human, because that is the way Dean thinks, and this is a Cass created according to what is in Dean’s mind.
Our other major example of an ungraced angel is Anna. When we meet her, she is able to tune in to “angel radio”, just as Cass is in “I Think I’m Gonna Like it Here.” In “Heaven and Hell”, both Pamela and Anna refer to being graceless and fallen as being or becoming human. But, again, it is not clear that this is not a simplification. Anna is mortal; she has a human body; for most intents and purposes she would seem to be human. But she has knowledge and abilities that a human would not have—telekinesis and “angel radio”, for example. And when Anna retrieves her memories, her first response is not to say that she used to be an angel, but that she is an angel. It therefore follows that a degraced angel is still an angel, though “human” is a common shorthand for this state. This is perhaps because an angel without her grace and wings is forced to take on many human characteristics (mortality, bodily and spiritual desires) and therefore thinks of herself as essentially human although she is not. perhaps in some ways similar to the way in which a desouled human is still human (but with rather different consequences.)
Grace seems to be possibly more-or-less the soul equivalent in angels; a source of their powerful abilities. (And if the human soul is not precisely a equivalent, it is a useful analogy.) It is apparently not their true form (given Zachariah’s self-description in “Dark Side of the Moon”, or Cass’s “My true form is approximately the size of your Chrysler building” in “Family Matters”.) It therefore follows that angels without their grace retain their true form, but remain folded into a human vessel, unable to change or leave without their grace. Removal of grace forces the angel to take on various human traits; most significantly mortal needs (hunger, sleep, avoidance of pain, and the like.) Yet they are still essentially angels, perhaps in some ways similar to the way in which a desouled human is still human (but with rather different consequences.) The angel is still an angel, still a member of that species, not a human being—even though they are occupying a human body. If grace is like the soul, saying Cass is no longer a member of the angelic species because his grace is gone is like saying Sam was no longer a member of the human species when he was soulless.
There is no reason to believe that species-switching can occur between humans and angels. Angels cannot become human—nor do humans become angels. We have never seen an angel with human origins. Likewise, we have never seen an angel truly become human. (A human who takes in an angel’s grace becomes that angel’s vessel; they do not become an angel themselves, as Anna did when she regained her grace.) A degraced angel may be described as “human,” but this seems to be a simplification, an approximation, not the true state of being. The relationship between angel and human is not the same as human and demon; humans become demons and demons can be “cured” and become human again because they are essentially still the same species. This is why a demon can forcibly take a human vessel while an angel must obtain permission.
Angels cannot become human, only human-like. Unlike demons, they are a different species—a separate creation. Castiel is an angel with a vital part of his being forcibly removed from him; a disabled angel, if you will. Should his grace still exist—which likely depends on whether it was the catalyst for or an essential part of Metatron’s spell—and should he regain possession of it, he ought to be able to regain his full angelic nature by reconnecting with it. Were he truly human, that would not be the case. He’s not a human being. He may live and die now in the same way that a human would—by force or by choice—but he is not and never will be human. He is still an angel.