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.

Large, beautiful breasts that are keep well consealed and initially appear to be much smaller and less impressive than they actually are until unleashed.

butt buddy, gay lover, etc