Allison Williams is an American actress, comedian, and musician. She's the daughter of Jane Gillan Stoddard, a TV producer, and NBC Nightly News anchor Brian Williams:
Allison stars as Marnie Michaels on the HBO television series Girls. (more at Wikipedia and IMDb)
Photos in this gallery come from Reddit and Imgur.
Happily! She's excellent on Girls and she can sing -- see the videos below...
There's a joke that Allison Williams is actually 50. Proof is Cheryl from Jake and Amir:
Allison Williams singing Hallelujah:
...and a few more videos of her singing from that time.
For Kurt Hugo Schneider fans, these were some of the first songs he produced, back when Sam Tsui was doing his cover of Michael Jackson songs.
Allison Williams singing the Mad Men theme song:
Allison Williams Marnie Michaels gifs from Girls:
Allison / Marnie singing What I Am: