She was the mistress (and muse) of poet and generally amusing troublemaker Guillaume Apollinaire; she painted this group portrait (which also includes Picasso, bottom left) with him at the center (and herself the smiling odalisque bottom right).
After Apollinaire, she married a German (not a brilliant idea early in World War I, if you were a French woman), but she soon saw reason and returned to Paris.
This portrait by Rousseau (from her Apollinaire period) sees her a little less faun-like than the photo would indicate.
She is considered the main female painter of the Cubist circle, as is clear from the self-portrait above.
As time passed, though, their influence seems to have diminished considerably; she came to specialize in willowy, languishing pictures of women in various stages of deshabille.
Still, at its best, her work is, I think, quite beguiling. Gertrude Stein claimed that in fact she was just painting what she saw - because her eyesight was so bad, the whole world actually was vague and pastel-colored.
Whatever the case, she was, as far as I can see, one of the very last painters who could convincingly paint a classical nude, and that has to be worth something...