South Park Headcanons

Marcy and Heidi

Marcy was a character in the earlier seasons of the show. She was a member of Cartman's cult and lost her father to a falling wooden beam in Cartman's poorly cobbled together church when he confronted Eric Cartman about manipulating her into his cult.

Heidi Turner on the other hand was Cartman's girlfriend for two seasons and went through grave personality changes due to his manipulation.

Marcy is said to be the same character as Heidi and her different voice as well as a different pair of parents are simply an inconsistency.

However, I like to entertain the thought of Marcy and Heidi still being two separate characters that simply happen to look alike.
Not only could they possibly bond over their identical appearance but also about the fact that they were both manipulated by Eric Cartman.
My favourite headcanon would be the both of them plotting a revenge plan to get back at Cartman for what he made them go through in the past. They would, however, not go to extreme lengths like Jenny Simons from my other headcanon would. They would just try to gaslight him to a point where he becomes paranoid and emotionally vulnerable but their attempts wouldn't bear any froots this soon because of their too mild nature.