It's Always Sunny In Philadelphia: This Theory About The Waitress Will Blow Your Mind
7. Charlie Is More Open With Dee Than Anyone Else
It's a well known (and utterly hilarious) runner throughout the show that The Gang find Dee totally and utterly repulsive. They mock her appearance, her comedy and see her nothing more than a barnacle stuck to their ship. Their friend-ship.
It's clear as day to see that Charlie does indeed join in with the mockery and supposed distain for her, but that's no different than a school boy mocking a girl along with his friends to hide his true feelings for her.
There are multiple examples of Charlie being sweet to Sweet Dee without a single insult being thrown around:
He gives her his green army jacket when she's cold in Underage Drinking: A National Concern. He defends her honour in Hundred Dollar Baby, Charlie doesn't attempt to drop Dee in The Gang Dances Their Asses Off, they share a candle lit dinner in Mac And Dennis: Man Hunters.
The evidence goes on and on about the relationship that Charlie and Dee have but the point remains, Charlie is extremely affectionate towards Dee when the two are hanging out without the judgmental eyes of The Gang.