Their first interaction was in the Season Nine episode "Marjorine", wherein Butters is sent to Heidi's girls-only sleepover, disquised as a girl named Marjorine, in order to steal a cootie catcher that the boys think is a future-telling device. On Marjorine's first day of school, Heidi, along with the other girls, isn't fond of "her", questioning her style of fashion. When Heidi's mom announces that Marjorine is coming over to her sleepover, Heidi is displeased on grounds that she didn't invite "her". As soon as Marjorine arrives, Heidi and the other girls made fun of "her" and force her to leave. This results in Marjorine locking herself in the bathroom to cry. Once Heidi sees this, she feels sorry for picking on her and agrees to give her a makeover to make her feel better about herself.
Their first real interaction is in the Season Twenty episode "Members Only", in which Butters goes to SpaceX to travel to Mars with Eric Cartman and Heidi. Butters then starts copying Cartman's "women are smart and funny" behavior around her and tells her jokes. He and Heidi appear to get along rather well, much to Cartman's annoyance.
In the following episode, Cartman thinks that Butters is trying to steal Heidi from him, but Butters tells him that it is the opposite since he doesn't want anything to do with girlfriends anymore after his ex, Charlotte, dumped him via Skype in "Wieners Out", implying that Butters neved had romantic feelings for Heidi in the first place and is just trying to be nice.
There are very few fanfictions that include this pairing.
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