Monster Tinder

[Amaterasu]

Sarah Kerrigan

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Spirit-Demon!

Let's be up-front here: Kerrigan doesn't need someone who she can get romantically, let alone sexually, involved with. She's probably got some love hidden deep in there beneath the chitin exoskeleton, but what she needs - if one can even speak of that - is someone who's useful to her, who complements her in some form.
The issue is, Kerrigan has just about everything she could ever wish for: she's intelligent, she's powerful (one of the most powerful creatures in the Koprulu sector, if not the entire universe, second only to Amon and the other Xel'Naga), she has an army of innumerable self-multiplying and adapting soldiers under her absolute remote control... the only things she could want would be more power, and if she ran into a monster that somehow has more power than her she'd just kill it and assimilate its essence into the Zerg swarm, or zergify it. I find this to be a bad thing because it would take away the monster's individuality. On the other hand, Kerrigan would perceive someone that cannot be zergified as a potential threat if they're powerful enough. I can't say for certain, but I imagine that Kerrigan's physical body is relatively vulnerable if caught offguard.
This leaves us with the following: It must be someone who's useful to Kerrigan in some manner, who's weak enough to not pose a threat or be an assimilation target and who's either unzergifyable or in some way dangerous to zergify.

Hence, a Spirit-Demon. To give you the jist, Spirit-Demons are the souls of deceased evil people who enter Hell and are transformed. Contrast regular Demons, which are "living" creatures. Spirit-Demons are nearly impossible to kill permanently because one would have to destroy their soul, which is more resilient than flesh and blood, though they can be temporarily scattered, taking a while to reform. They are fairly powerful users of demonic magic and, because they are a soul with physical form and not a meat-and-bones body, they are effectively immune to disease, broken bones, blood loss etc. - and Zerg infestation. Effectively, Kerrigan would be unable to kill Spirit-Demon or assimilate her into the Zerg Swarm. This will, of course, be a bad thing initially, though Spirit-Demon would be fascinated by Kerrigan's immense powers and wish to serve her - with the hopes of getting a little planet or two to reign over once the Zerg rule over the entire Galaxy, that is. And Kerrigan is a smart cookie: she'll recognize the use for a near-immortal creature, be it sending her on suicide missions such as for info-gathering where Spirit-Demon gets captured and finds ways to invoke the whole "Before I kill you let me reveal to you my master plan" trope, then gets "killed" and reforms to tell Kerrigan everything she knows.

I don't think that the relationship would go much further than that - Kerrigan would at best tolerate Spirit-Demon as a sort of ally, but it's unlikely that she'll fall in love. If she could do that she'd be together with Raynor. Similarly, Spirit-Demon is in it for personal gain, so... a mutually beneficial relationship would work? (This reminds me of Hiro's Ferengi...)