According to the I'll Be Your Mirror Songfacts, Lou Reed wrote "I'll Be Your Mirror" for Nico after Nico approached Reed with this line after a 1965 stage show. It is said to be a favorite of Reed's.
I'll be your mirror
Reflect what you are, in case you don't know
I'll be the wind, the rain and the sunset
The light on your door to show that you're home