Ria is one of the first characters we meet, along with her police detective partner, Graham Waters. We get to know his full name, but she's always Ria. Is it short for Maria? Victoria? Alexandria?
Whatever her full name is, Ria seems like one of the more level-headed characters in the film—on first impression. After a car accident, Graham spouts his weird philosophy about crashing into people to feel human contact. Ria's response?
RIA: Graham, I think we got rear ended. I think we spun around twice. And somewhere in there, one of us lost our frame of reference.
At least one of them is speaking sense, but that lasts for about, oh, four seconds. Pretty quickly, Ria starts spouting ignorant crap at Kim Lee, the woman who hit them. After Kim says "blake" instead of "brake," Ria—who, we must remind you, is a police detective—makes fun of her.
RIA: I "blake" too fast? I "blake" too fast. I'm sorry you no see my "blake" lights. […] Officer, can you please write in your report how shocked I am to be hit by an Asian driver!
After this outburst, Ria seems totally hypocritical when we see her defend her own racial heritage later in the film. She makes a good defense of herself, reminding Waters that she is half Puerto Rican and half Salvadoran—not Mexican. But it comes across as self-serving when she plays into other offensive racial stereotypes later in the film's timeline.
Other than that, Ria's character is pretty static. The most interesting thing is trying to guess what "Ria" stands for. Macaria? Valeria? Rumpelstiltskinria?