The prisoner's allegory
The novel frames the story as a kind of explanation or confession from a prisoner. The story can therefore be seen as an allegory of misfortune and mistake that should explain the reasons that this person is behind bars. The allegory works like an argument, so if the prisoner is evil and full of malice, then the reader might praise the incarceration, but if he is innocent, then his incarceration is unjust, and an argument can be made against the system. In this case, systemic injustice has its part, but mainly the problem is the complexity of life on the bottom of the socio-economic barrel.
The anti-parents
The poverty that afflicts Pascual's family is compounded by immoral and shady parents who abuse the children ruthlessly. The father is a patriarch in a criminal organization, and the mother is a follower of the worst kind. They don't set their children up to have better lives than theirs. Instead, they vent their negative energy into their children and steal from the kids whenever they have an opportunity to do so. They are the opposite of what parents should be, so they symbolize hatred and the way abuse limits the life of a child.
The sister and innocence
Rosario symbolizes a kind of experience that might be called innocence lost. She is a virgin when she decides that the chronic abuse and mistreatment of the home is unbearable. She leaves Pascual behind and pursues a new "career" as a prostitute, submitting herself to a pimp named Stretch. She escapes the horror of home by a compromise that makes her a martyr for a certain kind of suffering. Notice that the family now has all the major components of the criminal underground represented.
Mario's death
Pascual's loss of innocence is symbolized in another character, a young brother named Mario. According to a motif well-known from The Brothers Karamazov, the young brother is crippled from birth and is perfectly subject to the family's broken dynamics. The death of this child is the death of innocence in Pascual, and when Pascual struggles to feel sorry for young Mario, that represents the anhedonic response caused by chronic abuse. It isn't that he hates Mario or wants him to die, but rather, it is as if his mind has had all its pain receptors burned off by chronic abuse.
Stretch as a symbol
Stretch is clearly an important figure in the narrative, but what does he represent? Between him and the family father, two different approaches to crime are elaborated, one through prostitution and one through drugs. They both symbolize criminal enterprise in the poorest communities, but Stretch is a more clear symbol for the predatory nature of crime. He profits from the systemic disenfranchisement of women. It isn't like Rosario or Lola can go to college and become accountants or something; their decision to step out of their father's monetary support is a decision to accept life at all costs. Stretch is a stand-in symbol for the predatory nature of patriarchal systems.
