Personally ( and this is only because I found it true for me; and I’ve often thought maybe there should be another word to describe it) I find merit in Elizabeth Gilbert’s description:
People think a soul mate is your perfect fit, and that's what everyone wants. But a true soul mate is a mirror, the person who shows you everything that is holding you back, the person who brings you to your own attention so you can change your life.
A true soul mate is probably the most important person you'll ever meet, because they tear down your walls and smack you awake. But to live with a soul mate forever? Nah. Too painful. Soul mates, they come into your life just to reveal another layer of yourself to you, and then leave.
A soul mates purpose is to shake you up, tear apart your ego a little bit, show you your obstacles and addictions, break your heart open so new light can get in, make you so desperate and out of control that you have to transform your life, then introduce you to your spiritual master...
--Eat, Pray, Love