Who is T.J.’s houseguest?

T.J.'s houseguest is five-year-old Heidi Marshall.
In chapter 9, Georgia (T.J.'s therapist) tells T.J. that he is going to have a houseguest. 
Georgia mentions that Heidi will be staying with T.J.'s family because Heidi's mother, Alicia, made a mistake. Alicia left Heidi and her brothers with Alicia's abusive boyfriend, Rich Marshall, for an entire afternoon. Because of Alicia's actions, the Child Protective Services agency had to put a plan into motion to protect the children. Since the family taking in Heidi's brothers cannot take Heidi in as well, Heidi will be placed with T.J.'s family.
Georgia tells T.J. that she thinks he will be a good influence on Heidi. For his part, T.J. promises to look out for Heidi. Later in the chapter, Rich knocks on the door of T.J.'s home, asking for Alicia. Rich is drunk, and he begins to accuse T.J. of sleeping with Alicia.
During the conversation Rich reaches for his gun, but T.J. manages to wrest the gun out of Rich's possession. A little later, T.J.'s father comes charging out with a baseball bat, prompting Rich to leave. 
So, T.J.'s houseguest, Heidi Marshall, is staying with T.J.'s family for her own protection.