Castiel is an angel. He manifests on Earth inside his vessel, Jimmy Novak. He is the only angel that is shown to have been resurrected after having been killed, and he has been resurrected more than once.
Many angels laid siege to Hell to rescue Dean, but it was Castiel who ultimately pulled Dean out. According to Castiel, he did so “Because God commanded it.” His hand print was burned into Dean’s left shoulder. Castiel is amongst a group of angels who manifest on Earth to stop the breaking of the the 66 Seals which will release Lucifer and start the Apocalypse. Over time he develops an empathy for the Winchesters, in particular Dean. When he learns that Michael actually intends for Lucifer to be released so he can kill him, Castiel tries to warn Dean, but is punished. Ultimately he breaks allegiance with Heaven to side with humanity.
With Lucifer free, and Castiel cut off from Heaven’s power, he decides to try and find the long absent God. When he learns God is not interested in intervening to stop the Apocalypse, he is devastated. He works to support the efforts of Sam, Dean and Bobby in stopping the battle between Michael and Lucifer.
After Sam’s sacrifice to defeat Lucifer, Castiel tries to rescue him from Lucifer’s Cage, but fails to retrieve Sam’s soul. In an attempt to prevent Raphael from restarting the Apocalypse, Castiel is tempted into an alliance with Crowley. The demon offers to help find Purgatory which can provide them both with great power. His efforts mean Castiel becomes increasingly distanced from the Winchesters as he hides his efforts from them, to the extent of breaking Sam’s mind in order to hinder their pursuit of him.
After taking in the souls of Purgatory, Castiel kills Raphael. He claims that he is no longer an angel, but the new “God.” After smiting many people he considered to be doing evil in God’s name, he blacks out and loses control of his vessel, and his powers, to Leviathans possessing him. Conceding to Dean and Sam’s wishes, Castiel attempts to return all the souls and entities inside him to Purgatory.
After the souls leave Castiel, he apologizes to Dean, and pledges to find some way of redeeming himself. His plans are cut short when the Leviathans, who managed to hold on, take over his vessel. As the vessel rapidly deteriorates, the Leviathans march it into a reservoir, where it disappears beneath the water, releasing them.
Castiel is later found wandering naked in the woods near the water with no memory of who he is by Daphne Allen, who claims that God led her to him. She takes him home, he takes on the name Emmanuel Allen, and they marry. He discovers he has a gift for healing.
While searching for a way to help Sam, whose psychosis is killing him, Dean hears about Emmanuel from another hunter, and is shocked to discover it is Castiel. Castiel eventually regains his memories, and is horrified at what he did. He tries to heal Sam, but unable to do so, he chooses to take Sam’s madness into his own mind.
After becoming catatonic for a time, Castiel awakes crazed but cognizant when the Word of God is uncovered, and he helps Sam and Dean identify the tablet. He later aids Dean in killing Leviathan leader Dick Roman, and in the ensuing explosion is pulled into Purgatory with Dean, where Castiel disappears. A year later, when Dean escapes Purgatory, Castiel is not with him, and Dean tells Sam that Castiel “didn’t make it,” having “let go” in Purgatory when things “got hairy” not long before Dean escaped. Its revealed that in being in Purgatory restored Castiel’s sanity and that while he helped Dean escape, he remained out of his own free will as he chose to stay in Purgatory as his penance for his sins. However, angels led by Naomi rescued him and Castiel was rescued, in a somewhat weakened state and under orders that he is unaware of to aid the Winchesters and report into Naomi periodically.
Although he eventually managed to escape Naomi’s control, he was subsequently manipulated by exiled angel Metatron into completing a ritual that banished all angels from Heaven, the same ritual concluding with Metatron taking Castiel’s grace and leaving him a human. After some time forced to make a life for himself as a human, made more complicated by Dean’s decision to reject Castiel to take Heaven’s attention away from Sam, who was still healing from damage he sustained during the trials, Castiel was able to regain his angelic status by stealing the grace from the angel Theo.
Despite Metatron’s attempts to paint Castiel as the villain of the piece, he is eventually able to prove to the other angels that Metatron sees them as nothing more than fools to be manipulated and tricked, Gadreel’s sacrifice allowing Castiel to trap Metatron in Heaven’s prison. Although his stolen grace would eventually kill him, Castiel’s life was saved when Crowley killed the angel Adina to transfer her grace to Castiel. After curing Dean of his transformation into a demon, Castiel briefly reunites with his former vessel Claire Novak while working to resolve the conflict in Heaven. However, he is left on the run from Heaven once again when he extracts Metatron from Heaven to question him about a cure for the Mark of Cain. Even after taking Metatron’s grace to question him about a possible cure, Metatron reveals that he actually doesn’t know of a cure, but agrees to show Castiel to the location where he left the fragments of Castiel’s original Grace in return for his life. Although Metatron escapes with the Demon Tablet, Castiel is able to recover his grace and restore some of his power.
Source – Super-Wiki