While I'm not sure I can explain exactly how he's affected me, my best friend (and non-blood brother), Wirty, has had the most influence on me and my gaming style. Dude and I have spent many hours rolling dice. My characters are definitely more practical, worldly, and rogue-like because of playing with him.
The best part of our friendship? I can message him with something like "Quick, I need you to send me a sexy selfie" and I'll get something like this in less than five minutes.