Charlie is a womanizer, who dates both Jen and Joey. The former he is two-timing and they have mostly a physical relationship, which Charlie is not invested in. He seems like the type who knows what to say, but is about as genuine as I Can't Believe It's Not Butter. He does turn out to be insecure, but maybe not the most interesting person.
Charlie flirts with Jen at the gang's first Boston party, and Jen later bumps into Charlie at the college radio station. The two have an incredibly strong mutual attraction and quickly start dating, which entail mostly spending nights 'in' together, in Charlie's dorm room. Jen wants to get to know Charlie better, to substantiate their liaison, while he seems indifferent.
Mid-Season, he is revealed to have another 'girlfriend', Nora. She doesn't know that Jen exists either and when the girls find out, they work together to lock Charlie out of his room, naked. Both girls are sad about being betrayed, but find common ground and become friendly with one another. Later, however, Jen finds out that Charlie's been leaving Nora love letters and she is devastated.
Later, dating Joey, he starts having genuine feeling for; including serenading her in front of the dorms. Which causes conflict whether he should quit his band to stay with her or drop out of school to go on tour (S5;E20 Separate Ways [Worlds Apart])