Contreras is a member of the NCR supply corps currently assigned as Camp McCarran's quartermaster. He administrates the supply depot out of a makeshift shack erected by the McCarran Concourse. He is not entirely faithful to NCR, as he uses his position for personal gain, trafficking guns and chems to third parties.
After killing Contreras you can loot anything you may have sold him, as well as his caps.
Any items or weapons sold to Contreras are added to his personal inventory instead of his items stock, meaning he will equip the best armor in his inventory, and the best weapon he has ammunition for. This can lead to some odd outfit choices from Contreras, such as Legion faction armor, or Fiend helmets, while wielding anything from a Super Sledge to a Minigun (if he is in possession of 5mm rounds.)
In addition to many other chems, Contreras is the only merchant in the Mojave Wasteland who sells ultrajet.
If the player made the Science check when getting the supplies from Blake, he can blackmail Contreras into giving additional 100 caps on top of the regular reward for the quest.
He usually sells massive quantities of the rare 12.7mm rounds, usually within the thousands at mid-high levels, and usually sells a similar amount of surplus 5mm round ammunition. He also sells large amounts of .308 rounds.
If Contreras is arrested he can still be traded with; he will just be moved to the jail within the camp. He can then be killed without repercussions.
Contreras never seems to update his inventory or his cap amount before he is imprisoned.
When speaking to Contreras, after you ask him what he has for sale, and exit the sale box, the audio will play, but his facial expression will not change, the subtitles will not change, and no dialogue box will appear. Pressing the select button will allow the player to exit the dialogue. This will happen every time he is spoken to.
If you kill Contreras and then turn him in to Boyd his body would be in the jail cell.