There are a lot of things I love about this show, but probably my favorite aspect has to be this guy:
Guys, meet Castiel, usually called Cas for short (played by Misha Collins). He's an angel. I mean like an actual angel of the Lord, not the "did it hurt when you fell from heaven? *wink wink*" kind of angel... although he is that to ;P (-"You're an angel." -"I'm sorry, is that a flirtation?" -"No, it's a species." Lolz, just had to throw that in there).
He first appears in the fourth season and I think was only intended to stay for a couple episode story-arc, but his character was so positively received that his role got expanded. And voila! He's still around now (season nine having just wrapped up).
It's funny, I survived just fine for three seasons without him, thoroughly enjoying the show. But now, any time there's an episode without him I'm just like...
Anyways, where to begin? Well, I believe he was brought into the show as a way to introduce Christian themes and theology. Like, it's had demons with the holy water and stuff since the very beginning, but that's about as close as it got. Cas is the first angel we meet... And angels are JERK-WADS! Holy crap! I'm not here to describe every teeny-tiny little bit about the show, but just know that it twists everything you know about angels and heaven and whatever around.
OK, so the angels are puke buckets. Cas is an angel. Why do I love him so much if I hate the angels?
Well, since he was originally intended to only stay for a few episodes, he only appears randomly throughout the season without a whole lot of character development. He just appears "OK, you have to do this now. I'll help if necessary." Then once it's done, he's gone again. However, by the end, the brothers realize they're being used by the angels and start standing up for themselves, yadda yadda (I won't go into detail), but they manage to convince Castiel to side with them and he goes against Heaven's orders to help them. And a friendship gradually begins.
I say "gradually" because angels are incapable of feeling any type of human emotion. Also they really have no free will of their own. Well, actually they do, but they have no idea how to use it. They are like lost puppies who constantly need orders from a higher superiority. So the fact that Castiel rebels shows that he has some emotion and also now he has free will.
This is where the greatness of his character comes in, once he begins interacting with the Winchester brothers more, and encountering Earth and human life more. He is clueless. Like, he is very knowledgeable about science, history, theology, astronomy, anatomy, supernatural things, etc. but when it comes to interacting with people or pop culture or making jokes or understanding sarcasm, he has no idea. And THEN, for the longest time he has no idea that any of this is an issue. Gradually, he begins to pick up on it and starts trying a bit. Watching him try is absolutely hilarious ("WHY DID YOU KILL YOUR HUSBAND!?!! ...I was being bad cop." "You were being bad everything."). It makes it even funnier because he's totally serious about everything. Like I said, angels don't really feel any emotion.
You MUST watch this. A perfect example of Cas's ignorance to how human
life works, but he tries anyways. I think the background for this is that he upset
Dean (and maybe Sam too) so he decided to go shopping for him to try and
get back in his good books, if I remember correctly. So you see him just grabbing
random things he knows humans like, but also things he knows Dean likes XD.
However, over time, as he becomes closer friends with the brothers and gets more in touch with humanity, he begins to display a bit more emotion than the average angel. He most often shows affection and protectiveness towards the brothers (particularly Dean). He also frequently expresses regret, hesitance, and anger. Beginning before he rebels, he starts to have an internal struggle about what is right and wrong, instead of just doing what he is told. As the series progresses, he completely gains his free will and the ability to use it, and becomes the angel who is most in-touch with humanity.
So, I loved watching his development as he got in touch with his conscience and emotional side (that one episode when he was being controlled and was ordered to kill Dean, but Dean managed to break through by getting in touch with his heart made me squee and cry into my pillow). But I still totally love watching him struggle to figure out human interaction and pop culture. I guess I should say loveD now, because he recently had the most... well I'm not sure if it was heartbreaking or wonderful yet, but he had some major character development. He gained knowledge of ALL humanity's pop culture, just like that. With the touch of a finger. *sigh*. No more watching poor, naive Castiel. On the other hand, it's kind of amusing seeing him understanding ALL the references now. Like he had a plan to infiltrate a guarded base and when asked how, he replied, "We'll Wookie them", meaning we'll bring a fake prisoner. Bahaha! I couldn't stop laughing.
I haven't mentioned yet, he's a total BOSS too! Angels are like the most powerful beings in the universe. Practically nothing can kill/repel/trap them and there's almost nothing they can't do. They can heal, they can... teleport? I guess that's what they do, they can smite (placing their hand on one's forehead to kill), they have super strength, they can travel in time, they can turn invisible, they can pull souls out of Hell, and much more. Also, they are all warriors, so they are very capable fighters and all carry what's called an Angel Blade that can kill basically anything (and it is one of the few things that can kill an angel). So all of this applies to Cas.
Plus he's a little cutie. Just look at his puppy dog face! You gotta love him!