Saying that Rigby was always a jerk to Mordecai just isn’t true. Yes, he slacks off and gets Mordecai into trouble, but he isn’t always a jerk.

The episode Lift With Your Back basically goes over this, with Mordecai pointing out in the beginning of the episode how unhelpful Rigby is by mentioning previous events like not setting up the chairs and summoning The Destroyer of Worlds, messing up Mr. Maellard’s limousine with his meatball sub, and messing up the Benson portrait with his pretzel.

Later on in the episode, before it ends, Rigby points out the times he’s helped Mordecai’s problems, like how he’s basically the reason Mordecai knows CJ, how he helped Mordecai not have that awkward kiss with Margaret, and how he went with Mordecai to return Margaret’s sweater despite telling him to move on from the past. All of these things are things Rigby did because he’s Mordecai’s friend.

If Rigby really was always a jerk to Mordecai, why would he apologize for forging the College U letter? Why would Mordecai forgive such a horrible person? To say that Rigby always was a jerk to Mordecai is just wrong.