They are literally just IRL friends who decided a one-sided romance would be a funny gimmick. It was probably something as simple as Kiara saying "damn your l2d is hot, can my character lust over her" and they went from there.
I can't rule out that Kiara wouldn't want to fuck Mori IRL since she does seem pretty bisexual and Mori has a 10/10 body, but she obviously isn't trying to make something serious happen IRL. The entire thing is Kayfabe, Kiara getting depressed over her "unrequited feelings" was scripted since debut, and hell Mori even implicitly acknowledged that she's playing a tsundere character in the recent karaoke.
The funny thing is that people keep saying it's fake and therefore shit, yet at the same time swallowed the rrats that Kiara really is in love with Mori and that Mori really does get annoyed by her. I guess that makes them pretty convincing actors then, huh.
From what they've told us they already met before debut and went to karaoke together. Even aside from that they did tons of goofy stuff like singing cringe songs together in a restaurant which I don't think you would do with someone you didn't feel close to. Based on Kiara's comments however, I don't think they knew each other before applying to hololive which means they just instantly grew very close when they met. Basically they are close friends beyond just "co-workers who happen to live near each other".
Anyway their friendship is god tier, they do kinda remind me of Turk and JD. It's really sweet how much they do together. And so frequently too. Even the actual stuff we know about is already a lot, now add to that the tons of times they meet off-stream that they never even mention on-stream except in Kiara's SC tangents. Like today Kiara mentioned she met Calli AGAIN for unknown reasons.
The thing that I find most interesting about this is that they're completely different people IRL. People say opposites attract but that's a fucking meme and almost never true. Yet those two vibe hard together.
