The unnamed hitchhiker is described as "kind of a skankylookin little old girl" (8.2.395). We can't get much more colorful than that, can we?
Llewelyn picks up the 15-year-old runaway while he, too, is on the run. Llewelyn, who says "bad girls like bad boys" (8.2.23), thinks that he is a bad boy because he's on the run with 2.4 million dollars in drug money, and he thinks that this hitchhiker is a bad girl because she secretly wants to see a shootout. And maybe she does. But she doesn't think of consequences, like how if there's a shootout, she might get shot and die. Which is exactly what happens.
Why does Llewelyn think these things? Well, our little hitchhiker does hop right into the car with him, even though she thinks he's sketchy. When he says, "Reach me that machinegun from under the seat" (7.2.147), she doesn't bat an eye. She's either willing to go along with him because she's desperate to get away from home, or she wants adventure, or both.
Shortly before they're both killed, Llewelyn tells this girl, "If there is one thing on this planet that you don't look like it's a bunch of good luck walkin around" (8.2.315). Yeah, Llewelyn should have heeded those words, because she sure isn't good luck. We don't even get to learn her name before the drug dealers track Llewelyn down and kill him and her in a shootout at a motel.
The worst part of all this is that Carla Jean thinks Llewelyn was having an affair with the girl. He wasn't, but it looks suspicious. Plus, he married Carla Jean at sixteen, so this girl seems to be in Llewelyn's age bracket.
If you ever need a reason not to hitchhike with strangers—this girl is exhibit numero uno.