Sadna serves as Nailer's surrogate mother throughout the novel. She's a large woman who has taken care of Nailer since his own mother died and his father turned to drugs and violence. Her protective nature is one of her great strengths; she tries to shield Nailer from his father's drug-induced fits, and when Nailer, Pima, and Nita are held hostage by Richard and his crew, Sadna arrives with her heavy crew to save them.
But Sadna's also callous and calculating; she wouldn't have survived this long without weighing all her options. So after she saves Nailer, she asks him repeatedly if he really wants to leave with Lucky Girl (Nita), saying:
"You run and Richard Lopez will hunt you forever. You can never come back." She looked down. "You can still make a peace. Broker a deal and sell the girl to those people down there, and Richard will forget. You don't think so, but money will make him forget plenty." (14.119)
And this is the truth. We get the sense, though, that Sadna doesn't really condone this position, she's just saying this so that Nailer knows his options. This is reinforced by the fact that she ends up risking her own life to save Richard Lopez's worthless hide in the storm that destroys the tankers and clipper ships—so although she may talk tough, there's a streak of compassion in Sadna that runs deep.
Although Sadna is a physical force to be reckoned with, she also offers Nailer some much needed advice when he, um, needs it. When he kills Blue Eyes and saves Sadna's life, he's pretty torn up, and Sadna helps him understand the costs of what he's done. She explains:
Killing isn't free. It takes something out of you every time you do it. You get their life; they get a piece of your soul. It's always a trade. (14.58)
This sounds like advice from a woman who has killed and wrestled with the aftereffects. And when Nailer is trying to understand how he feels about killing his father at the end of the novel, it's Sadna who helps him untangle his emotions. She tells him:
"Getting over something like this takes time. It won't be better today. Not tomorrow, either. Maybe in a year, though, it won't be like this. Maybe in a year you'll have mostly forgotten. But it will still be there. You've got blood on your hands." She shrugged. "It always costs. It never goes away." (25.22)
That's the value of Sadna to Nailer. She's able to help him cope with what life throws at him when he needs a real parental figure, not his father, who's no kind of role model at all.