He's from Phoenix and has a sister named Sarah. At some point, for unknown reasons and under unknown circumstances, Danny became part of a sex trafficking ring. This included his traffickers planting a chip in him in order to locate him. They also provided him with heroin, which he became addicted to, in order to keep him in the circle and prevent him from leaving.
In the Series
Danny is brought into the hospital after being found passed out on the street. He is treated for a bowel obstruction but Dr. Charles becomes interested in his case when he flinches at the touch of Dr. Choi who tries to check his injuries and asking the staff when he could leave as he needed to.
Sarah Reese sits down to talk to him and he opens up to her a little bit. During the conversation, he begins to throw up from heroin withdrawal.
His 'mother', Tina, shows up and insists on Danny's immediate release. She refuses to leave his side and Danny becomes quiet as she is the only one who talks to the doctors. She takes Sarah's card away from Danny and says that he's booked into a rehab facility which is why they need to go. However, Danny signals that he doesn't want to go by coughing heavily. They order a chest X-ray where they find that he has a tracking chip implanted in him. Daniel tells Sarah that all the signs he's seen point to him being sex-trafficked but before Sarah can go and talk to him, his pimp pulls him out of the hospital. (Brother's Keeper)
He follows Sarah around the city, asking for her help outside of the hospital. After taking some advice from the legal department and Daniel, she agrees to give him help outside of the hospital during which she explains to Danny that there's help available if he wants to get away from his traffickers. After their first meeting, Danny is brought into the hospital having cut himself open and asks for Sarah. He says that he wants to take up their offer of help and that he was trying to cut out his chip, which Sarah throws away. (Alternative Medicine)
He comes back complaining of stomach pains, saying that he'd been kicked out of the homeless shelter he was at in favor of a woman and her child. Sarah tries to get him to report his pimps/captors in order to get him into protective custody, however he refuses to say anything about them and hence has nowhere to stay. Sarah tries to get him admitted into the hospital but he asks to stay with her instead. She initially rejects the offer but eventually decides to let him stay at his, but before she's able to tell him this he runs away. (Inherent Bias)
Danny is found dead a short time later. He was beaten to death, presumably by his pimps/captors who he ran back to after he had no where to stay. (Free Will)
Tina (His Pimp)
Tina comes into the hospital, claiming to be his mother. Danny becomes quiet when she's around and refuses to leave Danny's side. She grab's Sarah's card which she offers to Danny, insist that they leave before they do an x-ray, and lies about him being booked into a rehab center. She eventually gets Danny to sign a release form before they get the results from his chest x-ray back
He develops a close relationship with Sarah after she sits down with him just to talk due to Daniel insisting on a psyche consult. Following this, she earns his trust and he begins to open up to him. When he cannot talk due to his pimp being in the room, he starts coughing heavily which leads her to getting an x-ray and finding the chip but he's forced to leave before she's able to go back to him. He asks her to help him but outside the hospital, takes her offer of help to get him away from his traffickers, and requests her as his doctor every time that he goes in. He asks her if he can stay at hers until he can go back to the homeless shelter but leaves when she says that she can't
Sarah sees his body in the morgue, seeing that he was beaten to death She blames herself for his death, believing that if she immediately offered her home to him then he wouldn't have run back to his pimps who (presumably) beat him to death.