If a Batman story I read 25 years ago is to be trusted, the mate in chess comes from the Persian word meaning death, also -> Spanish matador. Or then Arabic.
So a mate can also be the thing that kills you. That's something to think about.. a soulmate... is the person who's going to devour your soul, it'll be the end of you. You will no longer be a single entity, two souls die and are born again as one when they finally meet each other. Whether that's really sinister or romantic depends on your viewpoint, I guess.