So Guerrero is one of Christopher Chance's pals in Human Target. Though, he isn't the main character, the way he thinks and acts is so intriguing that it deserves to be looked at. We don't know a whole lot about Guerrero's past. What we do know is that he did go to school (at least through the third grade, but probably all the way through high school as well). He we a tough kid even back in his grade school days, probably something he picked up from living in a rough neighborhood. From there, there's a significant gap between any other information, but we know that at one point he took some orders from Chance's old employer. We also know that even when Chance was still an assassin, he and Guerrero were probably friends. It's very possible the two were even partners from time to time. Still, since he can freely walk away from who Chance calls "the old man," it's clear that Guerrero doesn't have the same relationship with Chance's former employer that Chance or Baptiste did. Perhaps a weird twist is that Guerrero, who is famous among the criminal community for being the master of torture, has a young son that he checks on periodically.
Guerrero acts extremely apathetic most of the time, unless it has to do with his son or Chance. He does feel. He just never talks about his feelings with anyone. That isn't to say that he doesn't understand emotion though, like how Chance felt when Maria told him she was married. Guerrero was the first to tell her, "That's cold." He actually cares deeply about his companions, but he shows it more with how big a file he keeps on each of them.
He can be cruel. Harming people doesn't seem to bother him, even if the opposing party is a woman. His first mode of transportation for anyone whom he doesn't consider a friend is the trunk of his car. He, however, believes in the interesting idea of "democratic torture." He gave Ames such a choice the first time they met. He gave her a choice between peeling her finger nails and shooting her knee caps. When she pleaded, he threw the fingernail idea out the window, took out his gun and aimed it at her knee.
He doesn't trust anyone. He always demands that all payments for services rendered be in cash so no paper trail can be left behind. He's suspicious of banks, and loves cars.
Guerrero has a rare side that proves that he actually cares. When Winston was taken, he told Chance, "we'll get him back." And when they found Winston, when Chance took off, Guerrero took off the cuffs and told Winston that Chance was gone. He knew that the fingernail torture was gruesome with Ames, so he eventually threw out the idea. When Mrs. Pucci was asked by her sister-in-law about an "incident" that happened to her, mentioned by someone else, Guerrero covered for her. Mrs. Pucci had just had a terrifying and life changing run in with a homicidal manic who shot down her plane and had been chasing her and Chance for a full day in another country. After which, the guy followed her to the states and broke into her apartment, nearly killing her. When Chance arrived, she had just killed the guy in self defense. Instead of informing her sister-in-law about what really happened, he butted in during the middle of their conversation, simply said "some guy broke into her apartment." He said that what happened to Pucci was pretty common, and that was it. Her sister-in-law didn't press the issue.
At the same time he takes threats personally. When he was put in jail wrongfully by a bunch of dirty cops, he was barely kept from killing the warden that had put him in jail. And when a CIA member tried to get to his son when he couldn't get to Guerrero, he strapped the agent in his car and planted a car bomb inside it to make the man pay.
All in all, Guerrero is a unusually calm and complex man. But on the off occasion that he takes something personally, his opponent will pay.