The Soul Bro is that one and only best friend for you. They are the other half to your life of friendship.
<buddy>What do you say we go grab a few beers, golf 9 holes somewhere, and then do some fishin?
<you>Only thing better would be a few steaks, 2 of which are sittin in the fridge as we speak. Dude, you are my Soul Bro.
An individual who you have been destined to become best bros with. It is similar to that of a soul mate but to a higher degree and a more genuine manner.
Bubba: You are the perfect bro for me. We are similar in every way possible.
Forrest: Yeah, you're my soul bro.