COSMIC LOVER

Yor truly admired Loid since the very beginning, because of the way he treats her and Anya, and because of how reliable he is. She thinks he's a handsome man, who also has a cute side of himself, and is obviously attracted by his appearance and personality, yet I wouldn't call it love, but something like a crush. Yor never felt this way for another person before (yeah, she loves Yuri and Anya, but this is a completely different case), she has zero experience in this "field" and they've known each other for a very short time, so I don't think such a strong emotion as romantic love would grow in the span of a few months, but a crush is entirely plausible.

Loid, on the other hand, has plenty of experience on honey traps and physical demonstrations of "love", but never had the chance to truly open up to another person, to just be himself instead of this persona that Twilight is, and he's starting to feel that he can do it with Yor. Even though his spy senses sometimes tell him that he should be suspicious of her, he feels guilty when he does so and, at the end of the day, he keeps trusting her and being thrown out of balance whenever he's in her presence. Yor does "things" to him that his spy trained mind still can't comprehend, because he's used to calculate and plan for everything and, for a change, she's the most unpredictable person he has ever met.

In the beginning of the story, both of them were known for being cold and distant, completely detached from the people around them, but now they have each other (and Anya) and it all feels so new, that it's probably very overwhelming. So they're slowly getting closer, relying on one another, and their relationship is starting to bloom, even though it's still very incipient.

In my opinion, the first one to actually fall in love with the other, will be Yor. Like I said before, it already looks like she has a crush on him (and that's one of the reasons why she was so desperate about losing him – and, consequently, their family – to Fiona), so I think this will naturally develop into a stronger feeling as the time passes, but she'll probably keep brushing it off, thinking that it's just her growing admiration for him messing with her heart, and she won't realize it until some major event happens in the story (like Loid getting very injured, for example).

But if Yor will fall first, Loid will fall harder and he'll realize it faster, being the overthinker that he is. He already showed in many instances that he's letting his true feelings seep out from beneath his calculated actions (as Fiona herself noticed, when she went to their house), that he misses Yor when she's absent, that he worries about her well-being (all for the mission! *coughs*), and that he's growing attached to his family, but since Yor has already rejected his "romantic advances", he'll probably think that she doesn't want anything with him and will try to supress his feelings for her as best as he can (with no success, because he'll eventually cave in and admit that he's irremediably in love with her).