Spouse I would say. Your spouse is someone who could reflect you the most. Your habits, your emotion, your humor, your intelligence, your concept about the whole world.
you could never defeat him/her, even with your strongest power. Your tactic, wisdom, logic, calculation, physical are all useless. Even you take advantage for one round or two, you would get payback very soon. But the weird thing is that your softest part might conquer him/she, for a lifetime, bilaterally.