Mark lives with Byron and his mom because his own parents are dead. Byron "had been friends with Mark long before he came to live with (them)...(Mark) had lived down the street and it seemed to (Byron) that (they) had always been together...(Mark) was his best friend...(they) were like brothers" (Chapter 1).
When Mark's mother and father kill each other in a drunken brawl, Mark witnesses the violent confrontation. Significantly, the argument that instigated the fateful fight concerned the revelation that Mark is illegitimate; he is the product of his mother's affair with another man. In ominous foreshadowing of a basic defect in his character, Mark reacts to his parents' deaths with chilling indifference, even thinking, "this'll save me the trouble of shooting them myself". Then, thinking practically, he decides, "I can go live with Byron and his old lady". Since Byron and Mark have always been close, Byron's mom takes Mark in as a foster child (Chapter 8). The two boys are inseparable, running with a gang and getting into a continuous series of minor scrapes, until at age sixteen, Byron develops a sense of moral maturity while Mark does not.