Veronica has two specific forms. Her human form is that of a moderately attractive black-haired woman, who is slightly taller than Chef. She is often seen wearing a green dress held at the waist with a blue belt, and blue shoes. She is later depicted in a frilly wedding dress.
Veronica's true demon form however is that of a demented, massive beast of a woman; with low-hanging breasts, cloven hoofed feet, blood red eyes, protruding fangs, and large leathery wings. She does not appear to wear any clothing in this form. However, she has an exaggeratedly large tuft of pubic hair which acts as a loincloth.
Chef first met Veronica while searching for Kama Sutra books at the library. Using her hypnotic song, "The Morning After", she tricked Chef into spending the night, talking to him about "the goddess" and the evils of men, and forcing him to fall in love with her. The boys, however, grew suspicious of Veronica's behavior, primarily out of jealousy of losing Chef to her. However, after talking to Mr. Garrison, the boys realized that she was in truth a demonic Succubus, a demon sent from Hell to suck the life out of men. The boys soon discovered that Garrison was right, though Garrison wasn't speaking in the literal sense, when she revealed her demonic form. The boys attempted to warn Chef, who did not believe them and cast them out.
After doing research, the boys soon discovered that in order to defeat Veronica, they would need to force her to listen to her own song backwards. They did so at the wedding, causing her to reveal her true form to not only Chef, but to his parents and the rest South Park. After she managed to kill Kenny, Stan and Kyle sang enough of the song to thus cast Veronica to the pits of Hell from whence she came.