A mirror.

"People think a soulmate is your perfect fit, and that's what everyone wants. But a true soulmate is a mirror, the person who shows you everything that is holding you back, the person that brings you to your attention so you can change your life. A true soulmate is probably the most important person you'll ever meet, because they tear down your walls and smack you awake. A soulmate's 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 out of control that you have to transform your life."

-- Elizabeth Gilbert, Eat, Pray, Love

Eloquently said, and true in so many levels.