I'm not sure who exactly Ms Nelson is without more context. She could be a teacher, a storyteller in a community, or just an ordinary person known for her storytelling skills.
Given the title, Ms Shields is the key figure in 'A Christmas Story'. She could be an elderly woman with a lot of wisdom about Christmas traditions. Or she could be a young woman facing some challenges during Christmas time. Maybe she has a special gift or talent that is related to Christmas, like being an amazing baker who makes Christmas cookies for the whole neighborhood, but there's more to her story that unfolds during the events of the narrative.
The 'Cartman Christmas Story' typically focuses on Cartman's perspective of Christmas. In South Park, Cartman is a very distinct character. His Christmas story could include his thoughts on Santa Claus, his reactions to Christmas traditions. For example, he might try to rewrite the rules of Christmas gift - giving or try to take over the local Christmas events. It also might show how his relationships with his mother and the other boys in the town are affected by the Christmas season.
Well, a 'cartman dies fanfic' could be about various things. It might be a fan - created story where the character Cartman from South Park meets his end. It could explore how the other characters in the show react to his death, like Stan, Kyle, and Kenny. Maybe it shows the chaos or the unexpected changes in the town of South Park after Cartman's demise.
Yes, there are likely some out there. Some might explore different relationships he could have with female characters in unique storylines. Maybe they could be about a female character befriending Eric and changing his ways in some comical or heartwarming ways.
The main character is Cartman, of course. Then there are his friends. Stan, who is often the voice of reason among them. Kyle, who might oppose Cartman's crazy ideas. Kenny, who has his own unique presence. Also, Cartman's mother, as she can influence Cartman's actions during Christmas. There may be some other side characters from South Park too, like the mayor or local shopkeepers who are part of the Christmas setting.
I'm not sure specifically what that particular 'cartman x heidi fanfic' is about as there could be many different interpretations. It might be a fan - created story that focuses on the relationship or interactions between Cartman and Heidi from a certain show or fandom.