Why them?

He is an Angel. She is a demon. Good vs Evil. They are enemies but their attraction is undeniable.

Through Lucifer. Castiel fights him. Meg is on Lucifer´s side.

Unforgotten

5. Season

They meet. She looks at him, smugly, then talks to Lucifer. Castiel looks pissed.

Meg gives Castiel a nickname: Clarence.

He talks to her about Lucifer, distracting her while he loosens some screws behind her. A pipe comes down behind her, hits her, and Meg is thrown into his arms.
He tries to exercise her and fails. She laughs and taunts him. He looks as if he wants to kiss her but throws her into the fire instead and walks over her. She screams in pain.

6. Season

They meet again. Meg taunts him and flirts openly. Castiel looks angry and confused.

Meg kisses him to steal his angel sword. Surprisingly he kisses her back and baffles her.

Castiel saves her from Crowley, she smiles and vanishes.

7. Season

And they meet again. Castiel/Emmanuel can see her true demon face but doesn´t remember who she is exactly. Still Meg flirts openly with him. He looks scared and confused.

Castiel feels awkward around Meg.

Meg tells Castiel, who has lost his memory, that he is an angel.

Meg watches proudly when Castiel smites a bunch of demons.

Meg becomes a nurse in a mental institution to take care of Castiel.

Meg watches Castiel until he wakes up.

Meg calls him by his name - Castiel + Cas.

Dean and Meg argue - Castiel disappears.

Castiel calls her Meg.

Meg owns an angel sword.

Meg tries to tell the angels that she is not Sam´s lover.

Castiel makes conversation. Meg is irritated.

Meg kills the angel Hester to save Castiel.

Quotes

5. Season

Castiel: "You seem pleased."
Meg: "We´re gonna win. Can you feel it? You cloud-hopping pansies lost the whole damn universe. Lucifer´s gonna take over heaven. We´re going to heaven, Clarence."

Meg: "So, what can you do, you impotent sap?"
Castiel: "I can do this."

6. Season

Meg: "Remember me? I sure remember you, Clarence."
Castiel: "Why are we working with these abominations?"
Meg: "Keep talking dirty. It makes my meat suit all dewy."
Dean: "All right, simmer down."

Meg: "What was that?"
Castiel: "I learned that from the pizza man."
Meg: "Well, A-plus for you. I feel so clean."

Dean to Castiel: "I was gonna kill her (Meg) too. Of course, I´d have given you an hour with her first."
Castiel: "Why would I want that?"

7. Season

Castiel: "Her face. She´s...one of..."
Meg: "It´s okay. We come in different flavours."

Meg to Castiel: "Meg. I´m just here for moral support. I mean after all we go way back. Dean and me. Just met you of course. I think we gonna be good friends too."

Castiel: "This silence is very uncomfortable. Is there something I should know?"
Meg: "I don´t know. Dean?"
Dean: "No. Meg has that effect. Awkward. You know."
Castiel: "That must be difficult for you."
Meg: "Dean is making a joke, Emmanuel."
Castiel: "Oh." He smiles, slightly confused but also amused.

Castiel: "Oh gracious!"
Meg: "Damn it. Demons."

Meg: "You´re an angel."
Castiel: "I´m sorry. Is that a flirtation?"

Meg about Castiel: "I believe in the little tree-topper."

Meg about Castiel (while he is smiting a bunch of demons): "That´s my boy."

Meg to Castiel: "That was beautiful, Clarence."

Meg: "He's been like the naked guy at the rave ever since he woke up. Totally useless."
Castiel: "Will you look at her? My caretaker. All of that thorny pain. So beautiful."
Meg: "We've been over this. I don't like poetry. Put up or shut up."

Castiel: "Don´t like conflict." zaps away
Dean: "What the hell was that?"
Meg: "You heard him. He doesn´t like conflict."

Meg: "Let me just go and get my angel."

Meg: "We both call, who do you think Cas will come to? I'm guessing me. You heard him – thorny beauty, blah, blah. I'm the saint who stayed with him. He owes me. His words."

Hester: "Kill the demon and her lover."(Sam)
Meg: "That's not how it – we're not – "

Meg: "Yeah. Yeah, Castiel. It´s me." on the cell
Dean: "Cas? Where? Where is he?"
Meg: "Shut up."
Castiel: "I’ll stop speaking."
Meg: "No. No, Cas. You talk."

Castiel: "Meg, are you hurt?"
Meg: "Shut up!"

Meg: "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."
Castiel smiles.

Dean: "I couldn´t find Meg anywhere."
Castiel: "Yes, well, she enjoys laying low."