She's the actress playing Diane at the diner. See the coincidence of this still picture that was taken in another movie being similar to Mulholland Dr. 's last scene:

http://us.imdb.com/gallery/hh/0187811/HH/0187811/iid_1012887.jpg.html?hint=nm0187811

And here's Bonnie Aarons, also known as the Bum:

http://imdb.com/gallery/hh/0007491/HH/0007491/iid_979067.jpg?path=pgallery&path_key=Aarons,%20Bonnie

"Did they scoff the whole damn Smörgåsbord?" (Audrey) 

"Gimme a donut!" (Coop)

I agree.  Diane is so fragmented that she has multiple selves throughout the narrative, possibly even some male ones. 

It's just me, but I still can't help feeling that the psychic neighbor from the apartment complex and the derelict in the alley might be one in the same.

Got up early, found something's missing/ My only name/ No one else sees but I got stuck/ And soon forever came