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I headcannon that Karen and Kenny are great at fashion because of his awesome character designs in TFBW and TSOF. Basically, Karen helps him make his costumes and outfits when they go out places and tells Kenny all about what's popular in fashion currently. So, when Butters first comes out as Marjorie and dresses super weird, all the girls and Karen spend time helping her with her outfits. All the girls lend her clothes, and Karen invites Marjorie over to tell her all about what's popular. Kenny would basically watch a fashion show every time Marjorie came over to their house.
Karen would be like a teacher to them and would teach them both about women's fashion at the same time. Kenny would learn so much he'd start talking to Marjorie about her outfits all the time, and they'd bond over it. Kenny would even take Marjorie out shopping with Karen when he first got a job. Essentially, Marjorie would be really close to Kenny and Karen after coming out, and they'd all bond over women's fashion.