Why them?

He is an Angel. She is a Demon. Good vs Evil. They are archenemies but their attraction is undeniable. Loyal to their cause. Both are soldiers and love their father and want to please him. Both are hunted, lone wolves, who are outcast from their kin.

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 exorcize 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

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.

Castiel goes to Meg when he feels alone.

Meg calls Castiel her boyfriend.

Meg is concerned about Castiel's mental state.

Crowley threatens Meg - twice - Castiel moves to protect her - and Crowley detects the angel's affection for the demon.

Castiel leaves Dean alone in purgatory. Meg was caught by the King of Hell Crowley.

8. Season

Castiel heads to a hotel where demons are holding a hostage. Castiel kills the demons and frees the hostage. The hostage is Meg.

Meg explains what has happened. Every time Castiel speaks she looks at him like he is God's gift. Castiel looks awkward and confused.

Castiel beats Naomi to the order to kill Meg, because she knows too much, suggesting it himself. He cleverly distracts Naomi from what he really feels concerning Meg.

Castiel helps Megs with her wounds and Meg starts to flirt with him, both are reminiscing about their kiss in the past. They also discuss how everything was simple when the Apocalypse had started, he was good and she was bad, and how much things have changed between them.

Meg suggests to Castiel to order pizza and move some furniture around if they both survive this. Castiel understands what she is implying.

Castiel asks Sam to protect Meg. She is amused but agrees because Castiel points out the torture she had suffered through Crowley.

Meg and Sam talk about Sam's past with Amelia. Meg calls Amelia a unique person, because she kept Sam from hunting for a year. Meg calls Amelia a unicorn.

Crowley appears. Meg orders Sam to go and save his brother and her unicorn, meaning Castiel.

Meg fights Crowley. He stabs her with an angel sword. She screams and falls to the ground. Is she really dead?

Castiel disobeys Naomi and flees with the angel tablet. Does he know what happened to Meg?

9. Season

Fallen from grace and walking Earth as a human Castiel chose to call himself Clarence the one name only Meg used to call him to stay hidden from the angels. He doesn't use the name Clarence to introduce himself to April Kelly, the one he sleeps with.

Cute! He remembers her and likes the name she has given him!

Castiel pricks his finger on the thorn of a rose. All of that thorny pain. So beautiful.

Castiel babysits a baby girl. Oh dear. I can't wait for Meg coming to the house delivering the pizza.

Castiel ruptures all the thorns of the stem of the rose to use his own blood to draw an angel banishing sigil. Thorny pain. Yeah.

"It's Crowley. He can always do something." Castiel

"Crowley is always the problem. He's just waiting for the right moment to strike." Meg in 7x21

Castiel learned to think and talk like her.

Castiel: "I miss you, P.B. (peanut butter) and J. (jelly)." Castiel misses the taste of peanut butter. In 5x1 Dean complains that Meg's kiss tastes like peanut butter. So. Castiel misses Meg and her kisses.

13. Season

Killed by Lucifer and burnt by the Winchesters, Castiel wakes up in the Big Empty. A Cosmic Entity tells him that the Empty is the place where all demons and angels go to when they die and sleep for all eternity. The Cosmic Entity implies that he knows what Castiel hates, fears, and loves, and that there is nothing for him back on Earth. Is that because Meg is here at this very place?

15. Season

Once again Castiel calls himself Clarence the one name only Meg used to call him by. He still remembers and thinks about her. He loves that name.

Castiel goes to the Big Empty to search for Ruby and there is Meg, sitting on a throne, sipping from a glass of wine. Castiel looks happy seeing her. Just it isn't Meg but the Cosmic Entity using Meg's face and voice to mock and torture Castiel. He is not happy about her using Meg's favorite pet names for him, like Clarence, Feathers, and Pizza Man.

The abbreviation "I got wind of Castiel's angelic APB", could also mean Angelic Peanut Butter. So, Castiel smells like Peanut Butter, after the stuff Meg did taste like.

The Cosmic Entity still wears Meg's vessel, calling it the Queen's pretty face. Oh. Wow!

Castiel sacrifices his life, and goes voluntarily and with a smile to the Empty, where Meg is.

In the new heaven made by Jack and Castiel you got everything you could ever want or need or dream. So Meg will be with Castiel. He would never leave her behind in the Empty. The demon he loves, the one who took care of him and saved his life.

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: "Your God may be a deadbeat but mine walks the Earth." He did, didn't he, Meg?

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."

Meg: "You deal with him. I can't anymore."
Dean: "You might want to be more specific."
Meg: "I was laying low halfway across the world when emo boy pops up out of nowhere and zaps me right back here."
Dean: "Why?"
Meg: "Go ask him. He was your boyfriend first."

Crowley (to Meg): "Don't even think of smoking out, pussycat. I've got eyes all over the place."
Castiel: "Leave her be."

Crowley: "Oh, bonus. Meg, I'm gonna scoop you up, take you home, and roast you till you're jerky."
Castiel starts to move towards Crowley.
Crowley: "But not... yet. Cas can have you for now. Hilariously, it seems he'd be upset at losing you."

8. Season

Castiel: "The hostage is in there." Strange because he must be already aware who the hostage is. He arrived much earlier.

Meg: "But don't worry. I haven't exactly been giving them the Glengarry leads." She only looks at Castiel.
Castiel: "You mean you've been lying to them?"
Meg: "I just get them in the ballpark. Enough time's passed and enough's changed that they bought it."

Meg: "Someone kept picking up the trail and icing demons. I'm guessing that was you, Castiel. But Crowley just keeps sending more. He's hell-bent on..."
Castiel: "She's gonna...tell them the truth. Do I have to kill her?" Saying this Castiel looks like hell.
Naomi: "She does know the location of the crypts. But working with a demon is...unclean."
Castiel: "Well, we could use her - as Crowley did."
Naomi: "Agreed."

Castiel to Meg: "We need your help." Meg beams at him.

Castiel: "These wounds have festered."
Meg: "You really do know how to make a girl's nethers quiver, don't you?"
Castiel: "I am aware of how to do that. Although it doesn't usually involve cleaning wounds."
Meg: "Why are you so sweet on me, Clarence?"
Castiel: "I don't know. And I still don't know who Clarence is."
Meg: "Would it kill you to watch a movie or read a book?"
Castiel: "A movie, no. But a book with the proper spells, yeah, it could, theoretically kill me."
Meg: "You know, you're much cuter when you're shutting up."

Meg: "So, which Cas are you now? Original make and model or crazy town?"
Castiel: "I'm just me."
Meg: "So your noodle's back in order?"
Castiel: "Yeah, my noodle... remembers everything. I think it's a pretty good noodle."
Meg: "Really? You remember everything?"
Castiel: "If you're referring to the pizza man... Yes, I remember the pizza man. And it's a good memory."

Meg: "You ever miss the Apocalypse?"
Castiel: "No. Why would I miss the end of times?"
Meg: "I miss the simplicity — I was bad, you were good. Life was easier. Now it's all so messy. I'm kinda good, which sucks. And you're kind of bad - which is actually all manner of hot. We survive this... I'm gonna order some pizza and we're gonna move some furniture around. You understand?"
Castiel: "No, I-I -- wait -- actually... Yes, I--"

Castiel to Sam: "You should stay here and protect Meg."
Meg: "Since when do I need protecting?"
Castiel: "Since you were held captive and tortured for over a year." Castiel says this like THE MAN!
Meg: "Touché." She looks at him like "That's my boy."

Meg to Sam: "Go. Save your brother. And my unicorn." meaning Castiel

Meg to Crowley: "No Cas in the back seat. Your stone is long gone."

15. Season

The Cosmic Entity: "Hello, Clarence."
Castiel: "Meg."
The Cosmic Entity: "Oh, not so much."
Castiel: "You're the Empty."
The Cosmic Entity: "You always were quick."