Theresa Duvall was a muggle woman who ran away from her parents' home because she wanted to have her own life. She was only around 18 then, and her continuous arguments with her parents about her constant drinking and smoking was too much for her to handle, and she decided to bolt, consequently dropping out of college and effectively becoming, well, homeless. She decided to travel to London, seeking new opportunities, and that's where she started to get into the drug business because without a degree, she couldn't really get a worthwhile job - apart from bar tending, or waitressing, stuff that she she didn't really want to do. And so, she started out as a courier for dealers, meeting up with people and selling them pills, and weed, and other stuff. She soon became a dealer of her own, with a supplier and a network and everything, because of how much she earned the first time around. It's also around that time when she met Lucas Summers, another drug dealer, and together, they formed a relationship. They ran their drug businesses together, and earned a lot of cash, and they were happy for some time, before it all started going to shit, for Theresa, at least.
She got pregnant, and Lucas, afraid of commitment, decided to ditch her and travelled abroad, taking all of their business assets with him to the United States. Theresa was cheated on, and without anything, she couldn't follow him and she was essentially stranded in London with no money and a baby on the way. She got a measly job as a waitress just so she could support herself, as well as a run-down flat where she decided to live on. Trauma and deteriorating health deterred her from going back into the drug business, and so she was basically stuck down there, at the bottom of the food chain. A couple of months later she gave birth to her baby and named him Theodore, Teddy for short. She didn't really have that good of a maternal instinct and so oftentimes, Teddy would go ignored, crying and wailing for food and his mother's touch. Because of this, Teddy grew up resenting his mother, and they were a pair that didn't really love each other - with Teddy reminding Theresa of Lucas who plundered her, and Teddy hating a mother that never cared for him other than feeding him every other day.
Because of this, he learned how to fend for himself, and he became so independent at such a young age. He had a lot of run-ins with adults who'd take advantage of him, and so he started hating their lot in general - figures of authority, teachers, their neighbors, his own mother - because they kept underestimating him just because he was younger. When he turned 11, Teddy showed signs of magic, things that he himself was surprised of. Turns out, his mother's older sister was Muggle-born witch as well, and decided to fetch her nephew and enroll him in a school of magic, Hogwarts specifically. She had been keeping tabs on them ever since her sister ran away all those years ago, and when Teddy made some things levitate in the air, she just had to make her presence felt. Theresa, naturally, disagreed at first because she hated her sister, and so Teddy was delayed by one year. When he turned 12 and his mother still didn't want him to go, he got so angry that he made the cup Theresa was drinking from shatter into different shards. Fearful for her own life, she decides to send her son away to Hogwarts, and Teddy, who just wanted to get away from his mother, was ecstatic to explore the new world he just got himself into.
Teddy was a hatstall, but eventually got sorted into Slytherin, and is currently a fifth year.