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.