Charlie and Kate

Charlie and Kate have a friendship with benefits, as well as a therapis/patient connection.

Origin
The relationship began in the first episode, and it is implied that it had been going on for much longer.

Relationship
The relationship can be defined as a friendship with benefits. Charlie and Kate constantly engage in sexual acts, but feel no love towards each other. Charlie even goes as far as shouting "I don't love you. I'll never love you" during sex, but says it in a nonchalant way, and it is clear that they both reciprocate this feeling. Kate later becomes his therapist, but demands that he give up their sexual relationship if she does(due to it being unethical and against the rules). After a session, Charlie convinces Kate to continue their relationship because the rule exists to protect a patient from an abusive therapist, and they are both therapist, so the rule wouldn't apply.After a brief falling out over one of Charlie's patients, Charlie is given a choice between Kate and an apology, but he says that since she came to him, he knows that she feels sorry, so he doesn't need an apology and chooses her instead, but she leaves shortly after he corrects her about a famous therapist. Charlie and Kate also had to suspend their relationship so Charlie could date somone new. Ultimatly they broke up (just as Kate predicted) due to her strange beliefs.

Trivia

 * Ironically, Jennifer(Charlie's ex wife) says that she used to think that she(kate) had thing for charlie, but now she thinks kate is gay.
 * It was shown in Charlie Tries To Prove Therapy Is Legit that they have several rules for their relationship, and one of them is the ability to suspend their relationship if one of them is getting close to another relationship.