Will you look at her? My caretaker. All of that thorny pain, so beautiful.
As an angel and a demon, Castiel and Meg are sworn enemies on opposites of the battle for the Apocalypse. However, their complicated relationship has developed much throughout the series through working with the Winchesters.
Castiel and Meg meet face-to-face for the first time after he is trapped in the ring of holy fire by Lucifer, who asks Meg to watch him. With Castiel trapped, Meg proceeds to taunt him about how they are winning. Castiel plants seeds of doubt in her mind when he points out to her that Lucifer doesn't care about demons, which seemed to upset Meg greatly. As she steps forward to argue, Castiel telekenetically moves a pipe down onto Meg which knocks her through the ring of holy fire and into his arms. He then attempts to smite her but it doesn't work, causing Meg laugh at him. She taunts him about how he can't kill demons being far from Heaven, and asks him what he plans to do now. Castiel replies by throwing her onto the fire, and as she screams in agony walks over her body to escape from captivity.
Forced by Sam to work with Meg, Castiel agrees to work with Meg in order to find Crowley and retrieve Sam's soul. When the group is ambushed by hellhounds, Meg stays behind to fight them off. In need of a weapon, she kisses Castiel in order to steal his angel sword. As she pulls away, Castiel pushes her against the wall and kisses her again. Meg is stunned, and Castiel says that he learned that from the pizza man -- who is a character on a pornography film he found and watched earlier "because it was there". The rest of the gang leaves, and Meg faces off with the hellhounds. After the mission is finished, Dean teases Castiel, asking if he wants more time with Meg, but Castiel says no and is confused as why he would want to spend more time with Meg.
Meg and Castiel meet again, but Castiel is suffering from amnesia and goes by the name Emmanuel. When Castiel regains his memory, he transfers Sam's sickness to himself, which leaves him in a comatose state and necessitates his stay at the mental hospital. Meg poses as a nurse there and stays to watch over him, stating that she was loyal to Lucifer, not Crowley, and will therefore protect Castiel in order to screw with the former Crossroads King.
I know what I'm supposed to do. And it isn't screw with Sam and Dean or lose the only angel who'd go to bat for me.
Meg is seen reading a magazine at Castiel's bedside, when he finally awakens. He shows strange behavior, including his fondness of bees and his claim to not participate in any violence. He also grows attached to Meg and even remarks on her "thorny beauty" and asks her if she's okay. In the episode an angel, Hester, attempts to kill Castiel, but he is saved when Meg returns and stabs Hester in the back.
Meg is fed up when Castiel forces her to bring him to the Winchesters. When Crowley show up and threaten Meg, Castiel is defensive of her and Crowley leaves her alone for awhile because, as he points out, the Winchesters need Castiel to defeat Dick Roman and Castiel is fond of Meg and would be upset if she was taken away. When Dean yells at Castiel, causing him to leave briefly, Meg chides him for his actions and displays knowledge of Castiel's unique ability to tell the leviathans apart. Meg also stays to help the Winchester and Castiel in their plan to take down Dick Roman.
Go. Save your brother... and my unicorn.
Castiel rescues Meg from Crowley's demons and he bandages her wounds. While doing so, she flirts with him about the time they kissed in 6.10 Caged Heat. Meg indicates that when everything is over, she wants to have Castiel come round her house, move some furniture and play "pizza man" with him; he eventually gets her meaning, and when asked, says that he remembers their kiss, and he considers it a good memory. Sam and Dean, in the next room, wonder if they should trust Meg and Castiel referring to them as "Megstiel." While discussing Sam's leaving hunting for someone he loved, Meg understood how the right person could change someone, hinting at deeper feelings for Castiel. When telling Sam to go after Dean and Castiel, she refers to Castiel as "my unicorn" which is what she called Amelia Richardson after learning that Sam left the Hunting life because he fell in love. Meg ultimately sacrifices herself to hold Crowley off to buy Sam, Dean and Castiel the time they need and with her last words, taunts Crowley about Castiel getting away with the tablet.