Thank you to @RR-1357 for the original idea, I decided to do my own take on it. I am using canon details and trying to make it somewhat realistic, so if you see inconsistencies with canon in my stories, let me know. Beware, it gets somewhat dark near the end so proceed with caution. Hope you all enjoy!
Randy Marsh: Born to Marvin and Susan Marsh. Unfortunately, Marvin has actually struggled with depression much of his life, and it started when Randy was about three; this affected Randy later on. From the young age of six, Randy has had a passion for music; any music. He always wanted to make his own. When he was eight, Randy was diagnosed with a severely short attention span which may (and did somewhat) slowly go away with age. When he was eleven, he finally started taking guitar lessons and it was rather intuitive for him, and so he is still quite skilled with the instrument. When Randy was 16, he met Sharon in high school, where they fell in love. They got married 7 years later, and even later had Shelly and Stan.
Sharon Marsh: Born to Charlotte and Jared Kimble. Sharon actually had a very good upbringing. As a kid, Sharon was a bookworm and especially liked fantasy stories. As mentioned in one episode, she has been a big fan of musicals since she was a teen.
Shelia Broflovski: Born to Cleo and Michael Hikisam in New Jersey. Cleo and Michael were somewhat overprotective, and somewhat influenced Shelia’s parenting style later on. She has been an activist since she was fourteen. Ironically, one of her early causes was maintaining good relationships with Canada and avoiding war with them. When she was 25, she met Gerald. They married a year later and (a few years after that) moved to South Park and had Kyle, and somewhat recently adopted Ike.
Gerald Broflovski: Born to Thomas and Lola Broflovski in New Jersey. As a kid, like Kyle, Gerald was very good at sports. However, as a young adult he decided that he we better suited to work as a lawyer. When he was 28, he met Shelia.
Liane Cartman: Born to Harold and Mabel Cartman. As a kid, she was actually kind of like Butters. She was naturally a more soft, introverted kid, and her state wasn’t helped by her very strict, short tempered parents. (Note: I headcanon that their more tame behavior that was seen in canon is due to mellowing with age and mild senility.) This made her like Butters in school: Very sweet and lacking a backbone, and bullied as a result. This later fatally influenced her later life and parenting style. She also had a “dorky” interest in art and baking. When she was seventeen, Liane was sick of her overly strict and stern household, and so she ran away from home. Unfortunately, she had very little money and was without even a high school degree, and so she became a prostitute and a porn star and never got any better career. As explained in “Cartman’s Mom is a Dirty Slut” and “201”, she a few years later attended the Drunken Barn Dance of that year, and slept with many people, including Jack Tenorman. She got pregnant with his accidental child and so Eric was born.
Stuart McCormick: Born to Jonathan and Mary McCormick. Depression runs in the family: Mary’s father, Mary, and Stuart have severe depression. This also created a bad environment for Stuart. Due to both these genetic and environmental factors, from ages 12 to 19 he woke up every morning disappointed that he had not died in his sleep. Then, he met Carol. In his eyes, Carol was the most beautiful, witty, passionate goddess he had ever met. They quite quickly met, became friends, became much more than friends, and got quietly married. Upon marriage proposal, Stuart promised that together they would build their own house and have beautiful children. Carol would drop out of school and let him handle money making. Stuart would get a loan, get an education and get a good job to provide for them. Or so he promised. Unfortunately, he tried and tried to apply for a loan, but he couldn’t. After many attempts to make good money, he again became hopeless, felt rejected, and became a drug addict and an alcoholic. Carol went down with him.
Carol McCormick: Born to Jeff and Margaret Borev. Carol had a good upbringing, and was pretty chill and fairly well-liked as a kid. She was evidently on a good path in life, and decided to pursue a career in law. One fateful night, Carol (20 at this time) stopped her studying for the day and took a walk in the park, where she saw Stuart. In Stuart, Carol saw a beautiful, deep, loving soul where all others saw a rat with trashy clothing and stupid hair. They quickly became friends, more than friends, and husband and wife. They had three beautiful children, but it gradually occurred to them that it all barely mattered. The world apparently hated them, on account of the fact that them and their children had nothing and could get nothing. To cope with this and their young son’s constant violent misfortune, they turned to heavy drugs and alcohol.
Did you like these headcanons? If I made any mistakes tell me below. Also, should I make more?