Castiel (Supernatural)
Arrival on Earth
At the end of the third season of Supernatural, Dean Winchester is in Hell after being killed by the hellhounds of the demonic antagonist Lilith). In the season four premiere "Lazarus Rising", the angel Castiel is introduced as the one who brought Dean back from Hell and resurrected him. Because merely perceiving his actual form typically results in blindness, he takes a human host – James "Jimmy" Novak, a "devout man" who prayed for it – to communicate with Dean, and tells him that he has been brought back because God has work for him. Lilith is breaking the 66 seals in order to free Lucifer, and Dean must stop her.
Arrival on Earth
Castiel continues to appear throughout the season, at one point sending Dean back in time and later tasking him and his brother Sam with stopping witches from breaking another seal. He returns with fellow angel Uriel) at the end of the episode "I Know What You Did Last Summer", seeking to kill Anna Milton, a fallen angel with the ability to "hear" the communications between angels. In the following episode, "Heaven and Hell", Anna uses an ancient Enochian sigil) to send the angels away, though they manage to track them down later. Once there, Castiel expresses regret at having to kill Anna. Before they can carry out their duty, however, they are confronted by the demon Alastair) and two of his minions. A fight ensues, and Castiel is nearly defeated in the battle by Alastair. He is saved by Dean, and they in turn are rescued when Anna regains her powers. Although Uriel nearly vents his frustration on Dean, Castiel stops him, and the two leave.
Jimmy Novak alone
In "The Rapture", Castiel enters Dean's dreams and arranges a meeting to tell him something important. However, when Sam and Dean go to meet him, they instead find Jimmy, Castiel's vessel, who claims to have little memory of his life as a vessel. Anna theorizes Castiel must have angered his superiors, prompting him to be taken back to Heaven. When Jimmy is shot at the end of the episode trying to save his family from demons, Castiel returns, taking Jimmy's daughter Claire as his new vessel. After the demons are killed, a dying Jimmy begs Castiel to take him back as a host so his daughter will not have to go through what he did, and the angel acquiesces. Dean then asks him what he needed to tell him, but Castiel coldly replies his loyalty is to Heaven, not to mankind or to him.