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2.
Basically the same thing as Soul Sister, just referring to a guy instead.

Soul: The immaterial part of a person; the actuating cause of an individual life.

Brother: A male person who has the same parents as another person.

Soul Brother: Someone who fully understands you. Like a soul mate, but not someone you want to marry and make babies with. The brother of your soul.
Dude1: Brah! Why is it, that we like all of the same things, hate all of the same things, and have pretty much EVERYTHING in common?
Dude2: Because Brah, we're Soul Brothers.
by SheaBoogie June 02, 2010
 
3.
butt buddy, gay lover, etc
rufus asked his soul brother luthar if he wanted to be the pitcher or the catcher this time

lenny asked his soul brother ralph if he wanted to play cops and robbers tonight
by Mr. T 55555 June 17, 2007