Bitter-Root is a sniper with the NCR's 1st Recon at Camp McCarran. He is very unfriendly and does not like to talk; however, if told this, he says that he doesn't "mean anything by it", and that it's only his nature. His bitterness comes from his tragic past which he tells to the player.
As a child his parents were members of the Great Khans living in Bitter Springs. His father was a sadistic murderer with an alcohol problem who used to take Bitter-Root out shooting NCR (military personnel and civilians alike) since the age of three, later laughing with his fellow drug addicts about the killings. Bitter-Root's mother was an insipid person whose only useful quality to the group seemed to be her being, to quote Bitter-Root's father, "a smokin' hot piece of ass", attempting unsuccessfully to sell her son into slavery to score drugs, and likely whoring herself for the same purpose. He loathed his parents and the upbringing he received.
Bitter-Root was "liberated" from this environment during the Bitter Springs Massacre by the NCR troops and Major Dhatri (who was a Captain at the time), who took him in as his son - which is why he sometimes refers to him as his father. He claims he does not remember anything about the massacre but when pressed (skill check—Intelligence 7) he admits that after the battle the blood he was covered with wasn't necessarily all spilled by the NCR, and that perhaps he had some scores of his own to settle, implying that he burst out in a fit of aggression against the Khans himself (which may be the reason he was not killed by NCR along with the other children at Bitter Springs). and implies that he might have killed a few of the Khans himself during the confusion of the attack.
Bitter-Root's name is part of an old Khan ritual; upon reaching the age of adulthood all Khan children are severely beaten in a rite of passage in which they choose their own new name. Although too young to undergo the beat-down himself, he chose the name Bitter-Root and feels that the Bitter Springs Massacre was his ritual, and that the Khans "owe" him his name. His choice is a double entendre, referring both to his bitter roots (i.e., his parents) as well as to the traumatic event that he considered his rite of passage - the Bitter Springs Massacre.