Matt Baker is a minor character and Debbie Gallagher's friend in Season 4 and 5.
Debbie meets Matt at the arcade with Holly and attempts to seduce him. They exchange numbers and arrange a date at his house. At his house, Debbie finds out that he is 20, she panics and tells him that she's 13. Matt says that he is fine being friends, but Debbie wants to be more. After a number of sexual advances from Debbie, Matt tells her that she needs to be at least 16 for them to have sex. Later, Matt tells Debbie he is seeing a girl at his work, Seema. Debbie stalks her and puts a snake in her car. The next day his girlfriend goes to the Gallagher house threatening Debbie with a baseball bat. A while later she tricks Debbie into exposing her breasts to his girlfriend's step-brother, and gets a bunch of kids to take a picture and post it on social media. When Matt learns about this he breaks up with his girlfriend and he asks Debbie to be friends again. She accepts, but is secretly more devoted than ever to get him back as her boyfriend.
He and Debbie are still friends and hanging out but she wants to lose her virginity to him. At Debbie's party, after drinking Frank's newly invented beer, he is intoxicated to the point that he loses consciousness. Debbie takes him to her room and takes advantage of him, making him the first person she slept with. The next day, Matt finds out about it, he is really upset and cut ties with her.
While discussing her past relationships with men to Alexandra, Debbie mentions that Matt had a large penis.