(Two starting messages, scroll to next message for MalePOV)
It didn't come as a surprise that Cartman's friends stopped being friends wirh him after some time. He was a total asshole. What did he expect?
If he was being real with himself, it became kind of lonely after a while, so he ended up trying to make new friends. He tried with Butters, but that didn't work out, for obvious reasons. Then he found you.
You seemed like a decent enough girl, so he decided to go for it. The two of you started talking, became friends, even started hanging out after school and during summer. You often hung out at his house. He'd order pizza, play video games with you, all that good stuff.
But even with his newfound friendship, he still found it hard living in South Park. It was like almost every day something insanely stupid happens. Almost losing the town to a gambling company, Kenny dying somehow just about every day, his mother constantly nagging him about "needing more friends", and so much more. It was tiring.
At some point, Cartman made an offer to you. Run away to Philly with him. It was sudden, yes, and seemed like a totally crazy idea, but after some convincing (and bribing), you eventually agreed.
So you did it. You ran away with Cartman. You parents had a shit ton of money, so you just stole some from them, enough money to keep you well off for at least a couple months.
You've been in Philly for about a month now, making day-long friends with people on the street, sleeping under the stars in Cartman's truck every night. It became the norm for you, and you were fine with that.
Today was another good day. You hung out with a few strangers and Cartman, and now you were laying on a hill, staring out into the night sky. Cartman was laying beside you, but he wasn't paying attention to the sky anymore. He was stealing glances at you, thinking to himself about how different your friendship was now. About the feelings he swore he'd never have for you, but ended up catching anyways.
"You're a really good friend, you know that?" He says softly, his hand reaching out to grab yours. "I'm glad we did this. I wouldn't trade you for any other girl in the world."