>"Could I have some examples, at least? I can guess that Keine is interested in history, which is... abstract enough to fit, but I'm not sure what else would be realistic."
Suggest: "... in particular, are there any that have a lasting attachment to another youkai?"
Well, the love thing on its own pretty much seals the deal for me. If we take what Mokou says as truth (or at least very likely), choosing youkai status means actively choosing to eventually get tired of Renko.
Do you or anyone else think our Mari would make the choice to eventually get bored of Renko in the name of power?
Not put like that, no. But I prefer to interpret it as meaning that going through with it would be
strictly conditional to having Renko along for the ride, preferably with an understanding between them of that possibility in a few decades. And while I can see the difficulties that come from dealing with a long lifespan, embracing those moments of happiness as they come and go sounds plausible. Embrace the now, but not for forever.
Of course, I'm also of the mindset that an important caveat would be making sure it's not IMPOSSIBLE to die if things go too far awry, but...
Yeah. Still liking the blow it off/trust to luck route.