I think she's a vampire.
Definitely a vampire. Check out her neck and the tiny little pointed teeth just beneath her top lip. Glowing red eyes are a hint, too. Unless... she's a vampire elf. In which case, I don't know what to say and will question everything about reality.
Elf, vampire, it doesn't matter...the art is sexy.
It's Marceline! From Adventure Time.
I had to Google her. Yeah...that drawing is much better than the cartoon.