The following is based on Van Buren and has not been confirmed by canon sources.
Chagas is one of the vital escaped Tibbets prisoners. Like many of his escapee comrades, he carries one of the New Plaguemutations – a piece in a large and lethal puzzle that the Prisoner needs, and the same virus that inhabits the Prisoner's own body.
Chagas is the brother of Ramon, the previous leader of the Blackfoot tribe. He is good at using guns and tracking. Chagas was captured by some NCR troops during a raid on one of their caravans. The caravan had found one of Victor Presper's infected guys, and ODYSSEUS had the raiders and surviving caravaneers rounded up before they could take Chagas back to NCR for trial and imprisonment. Chagas became infected because of his exposure to the caravaneer and his time in the prison.
After the Tibbets prison was attacked by Presper's men, Chagas escaped back to his tribe. Now that he's back, he wants to run the tribe, just like his brother Ramon did before Chagas was collected by ODYSSEUS. He's not quite as skilled as Kurisu, but with luck he might be able to take her, and with the Prisoner's help on top of that it's a pretty fair match. If the player is smart, he'll get Chagas to agree to come back to ODYSSEUS first in exchange for help in taking over the Blackfoots. While he has been in prison for several years and is a little out of touch with his people, he does have pretty good mechanical skills and knows how to make simple one-shot guns.
Chagas was captured by Kurisu, so she owns him, and she's not going to let him go. Kurisu may have sold Chagas to someone else. The player, when asking about escaped prisoners, gets told about another prisoner instead of Chagas, since the Blackfoot don't consider Chagas to be a prisoner - he's one of the tribe. Once the player realizes they're chasing the wrong guy, they may come back to find Chagas sold off or missing.
Once the whole prisoner issue is addressed, you can set him up here as the tribe's gunsmith. He'd rather be leader, but this is a position of power and respect that a DiploBoy can talk him into.
Chagas has not appeared nor is he mentioned in any published game. He was to appear only in Van Buren, the canceled Fallout 3 by Black Isle Studios. It is not certain if he will appear in any future Fallout game.