Stuart "Fats" Wall is the adopted son of Colin and Tessa Wall, making him the son of Winterdown's deputy headmaster and guidance counselor. He is best friends with Andrew Price and bullies Sukhvinder Jawanda by commenting on her appearance in class and posting links about hirsutism and lesbianism to her Facebook page. He has a sexual relationship with Krystal Weedon, who later in the book hopes to become pregnant with his child so she and Robbie can have their own house away from her mother and Obbo. At Howard Mollison's party, he kisses Gaia Bawden while she is drunk, to Andrew's disappointment.
He is having sex with Krystal when Robbie Weedon wanders off, and he does not attempt to help save him when he falls in the river.
He is very concerned with the idea of "authenticity," which, to him, means being honest, true to one's own nature and never hiding or faking anything to seem nicer than you are (but not pretending to be worse, either). Ironically, his "authenticity" is mostly an image he has of how he wants to be. He holds his parents in contempt for not being authentic, and even thinks he might prefer Simon Price as a father, because he is an authentic person in Fats' view, and would be a worthier enemy than Colin.