Castiel is an angel that brings Dean Winchester back from Hell following his death at the hands of Lilith's hellhound. He tries to communicate with him twice after his resurrection, but fails to do so after incorrectly assuming Dean is capable of withstanding the power of his voice. He later meets with Dean within the body of a human host, and tells him that he had raised him because God has work for him to do. Dean is skeptical, but both Bobby and Sam believe the claim to be true, as there is no other being that they can find or know of that is capable of pulling a soul out of Hell and that is also immune to all the known methods of killing supernatural creatures.
Castiel later returns to congratulate Dean on dealing with the Witnesses. Although Dean is angry with him for knowing about the event but not assisting them, Castiel explains that he was busy on other battlefields. The angel then reveals that Lilith is releasing the 66 seals, one of which is the raising of the Witnesses, in order to free Lucifer.
Dean is later sent back through time by Castiel to when Azazel had made a pact with Mary Winchester to show Dean all that the angels know of the demon's plan. However, he admits that they are unaware of the main goal that he had for his psychic children. When Castiel returns Dean to the present, he tells him that they are unsure of Sam's part in Azazel's plan and that Sam is headed down a dangerous road. Pointing out that Sam, who had left in the night to go with Ruby, is not in the hotel room, Castiel tells Dean of his brother's location and gives an ultimatum: either stop Sam, or the angels will.
When the threat of the releasing of the demon Samhain arises, Castiel reveals to the brothers that he cannot allow another seal to be lost, having brought a "specialist" angel, Uriel, to smite the entire town in an effort to destroy the witch intent on resurrecting him. Dean refuses to allow them to do such a thing, and threatens to remain in the town. Castiel, already reluctant to carry out his supposed orders, allows Dean to do as he wishes. After the Winchester brothers manage to vanquish the demon, Castiel pays Dean another visit, revealing to him that their real orders were to follow his orders. He then tells Dean that he finds humans to be "works of art", being creations of his Father, and that he was praying that he would choose to save the town. He then confesses, in confidence, that he sometimes has doubt about distinguishing what is truly right and what is truly wrong any more. He leaves Dean with a warning that Hell would be brought to earth if they failed, and that he couldn't fathom the weight on the hunter's shoulders.
Castiel returns with Uriel seeking to kill Anna Milton, a fallen angel with the ability to "hear" all the communications between the angels. Anna uses a spell to send the angels away, though they manage to track them down later, having threatened to Dean that they would kill Sam. Castiel chose not to deliver this message, as according to Uriel, he liked Dean too much. Once there, Castiel expresses regret at having to kill Anna. She coolly replies to him that he couldn't feel the full scope of that emotion. Before they can carry their duty, however, they are confronted by Alastair and two of his demon minions. A fight ensues, and Castiel attempts to destroy Alastair while Uriel deals with the others. Unfortunately, Alastair proves far stronger than he imagined, and not only rebuffs the angel's attempts, but easily knocks him down with a single strike. The demon then begins to chant a spell of an unknown purpose, which seems to be potent enough to strike fear in Castiel. He is saved by Dean, and then all of them are rescued when Anna regains her powers. Although Uriel nearly vents his frustration on Dean, Castiel stops him, and the two leave peacefully.
Months later, the pair of angels once again appear before the brothers, who are in the midst of another case, warning them that another seal would be broken. The Winchesters manage to stop the ritual from happening, but get separated afterward, leaving Dean alone to be confronted by Alistair. Castiel arrives to save him, and captures the demon. However, even after several days of interrogation, they are unable to extract any information from him, especially on how the demons are killing angels. Unable to turn anywhere else, Castiel and Uriel are forced to ask Dean to torture the information out of Alastair.
Later, through mysterious circumstances, Alistair manages to escape the Devil's Trap, and nearly kills Dean before Castiel once again intervenes. The demon is then killed by a timely Sam after the angel is overpowered. Suspicious of the circumstances, Castiel later confronts Uriel on Alistair's escape and demands the truth from him. Uriel admits that only angels can kill other angels, and that he and others like him are now working together to free Lucifer, having grown weary of never hearing from God and jealous of His supposed favoritism for humanity. He asks Castiel to join him, but Castiel refuses and attacks him. Eventually Uriel overpowers him and gets ready to kill him. However, Anna joins the fight and kills Uriel. Later, Castiel visits Dean in the hospital, and tries to encourage the despondent Winchester by telling him he is the only person who can stop the Apocalypse. Dean remains unconvinced and tells him to search elsewhere for a hero.
 Powers and abilities Castiel projecting his wings to prove that he is an angel
As an angel of God, Castiel is an extremely powerful being. Merely perceiving his actual form typically results in blindness, as the appearance of his natural "visage" is so overwhelming that it is capable of burning an individual's eyes from their sockets, including those possessed by a demon. However, according to Castiel himself, certain "special people" are able to tolerate his true appearance and voice. Apparently, one can protect himself from such personal damage by shielding his eyes from the presence of an angel. Castiel appears to be capable of projecting at least a portion of his true self from his host, as at the end of "Lazarus Rising," he is able to display the shadows of his wings to Dean as proof that he is in fact what he claimed.
Castiel possesses astounding power, though his abilities are not completely limitless. He is apparently immortal, and is invulnerable to mortal harm. Although powerful demons such as Alastair can overpower him, only another angel would be capable of killing him. Similarly to a demon, he is capable of possessing a human and fully utilizing the body, yet he claims that his host is, or possibly was, a "devout man who prayed for it." He also possesses the ability to seemingly vanish and materialize from thin air. Though he typically seems to attempt to avoid fighting, he is a very formidable opponent when need be. He possesses the ability to kill or exorcise a demon simply by touching his palm to the forehead of its host, although the demon Alastair is immune to such an attack. Castiel usually deals with troublesome humans by touching his finger to their foreheads, which results in instant unconsciousness. He also possesses the ability to bend time, an ability he shares with other angels, allowing for the teleportation of himself and others through time. However, although actions of the time may be changed, their outcome will still be the same as they were before the time travel happened, as destiny cannot be changed. Castiel also has the ability to take a soul from hell, a feat that is difficult (if not impossible) for any being besides an angel. He also has the ability to resurrect said soul, putting them back in their body and restoring it to perfect health (no matter the condition the body was in before said resurrection), leaving only a burn in the shape of a handprint on the body. It has also been shown he has some level of telekinetic abilities as he caused a pipe handle to move when he moved his hand.
Castiel typically displays very little emotion and always exhibits an extremely somber disposition. He has not been observed expressing any strong emotions. According to the fallen angel Anna, angels possibly do not possess the ability to truly feel emotion. However, Castiel frequently exhibits what could be referred to as friendly affection towards Dean. He seems to at the very least come close to expressing regret, hesitance, and anger several times, and has once quietly laughed at a joke Dean made in "It's the Great Pumpkin, Sam Winchester", showing possibly a sense of humor. Collins feels that, through Dean, "[Castiel is] learning a little bit about humanity and re-learning something about [his] own humanity from that exposure. So I think that there’s a humanizing of Castiel going on."
Contrary to angels in series such as Touched by an Angel, Castiel and other angels are not out helping people in need. Though he has a conscience, Castiel is willing to kill innocents if the need arises. Speaking of his character, Collins stated, "I think that these angels are at least loosely derived from some Biblical angel stories, and those angels are [very tough]. They just destroy. I picked up Revelations, and they destroy, they destroy, they destroy. There’s no mention of cherubs and harps or any of that." In spite of all his apparently emotionless countenance, however, it is later noted that he is beginning to develop an attachment for Dean. This growing ability to feel even causes him to be "demoted", as his superiors fear that emotions are clouding his judgment.