Sometimes the best lighting for a portrait comes naturally through a window. We just need to proof it by looking these photos below.
Texture by Paige
Light Blonde by Duncan Harris
Do you see... by Adriane Dizon
image by Jean-Pierre B
0229 by CIA DE FOTO
...by the balcony. by Jason
"I don't want to stare out the window" by Eric Mueller
Be True To Thyself by Nicki Varkevisser
image by Katrina DeFrancesco
Source: http://www.photographyblogger.net/14-great-examples-of-portraits-that-use-window-lighting/