So, recently Francescu abandoned mostly-oblivious UST with Ken Hidaka in favor of a relationship. I will here attempt to explain why. Usual warning for UL spoilers and possible Hitherby digressions apply.
In my Big Damn Essay ("On Kindness and Affection"), I stated that Francescu had never connected himself with the idea of sex. Events, however, have been eroding that. The first kiss with Ken began the process; it forced Francescu to consider the idea that he, himself, could be found attractive by others. (In "On Kindness and Affection" I claimed that the kiss hadn't changed Francescu. This was not borne out by events, and I apologize to my readers for the misinformation.)
Francescu has never been good about analyzing his own emotions until or unless they hit him in the face. Retroactively, he probably started finding Ken attractive not long after the kiss. Realizing this took significantly longer; he saw it as a deepening of friendship, not having experience with attraction. It wasn't until he'd been hit with several extreme events in a row that he actually figured out what was going on within his heart.
Naked Day, although there was a fair bit of UST, surprisingly didn't contribute much to Francescu's revelations. He spent time with Ken because Ken was his friend and lonely, and their state of undress was pretty much irrelevant. The only thing it really contributed was adding the knowledge that Francescu enjoyed spending time with Ken.
The real revelations started when Francescu inadvertantly broke Evil Overlord Rule #34 ("I will not turn into a snake. It never helps.") He himself wasn't all that bothered by the transformation, accepting it philosophically as a camp thing and the result of tempting fate. Ken, on the other hand, was angry and upset; significantly more so than any of Francescu's other friends (the closest being Manfred). This caused two major reactions. First, it moved Ken completely out of the "apprentice" category; apprentices almost never worry about their masters, since the relationship is so unequal. (Francescu will still work with Ken to control his bezerkerness, but he's no longer a student). Secondly, it proved that Ken cared about and worried for Francescu, which ties into the section of "On Kindness and Affection" dealing with caring (in summary, anyone who cares about any scion of Gargamel and shows it more or less owns said scion.)
And then we get to the past post, which is when the relationship really started. Francescu witnessed Ken getting beaten down by Kase's goons and set on fire, and it hurt a lot more than Francescu would have initially expected. It pretty much ripped his heart out, in fact. Which made him ask himself "why does this pain me so, when I know it is only a memory?" The answer didn't become clear to him until Ken almost kissed him again, once they were out of the memory. Francescu realized, about then, that whatever he was feeling for Ken was deeper than friendship. The rest is...well, history.
Francescu has also been aggressive (for him, which really isn't very) in pursuing the relationship. This basically boils down to the fact that he's incapable of being casual about things like this, emotionally. It's Important and Serious and something he's realized he wants, and he's not shy about pursuing things he desires. (Ref: "Tomas vs. Francescu: Fight!" on what it means to have retained the ways of his childhood; he doesn't want much in the series, but he does fight for it.) There's also a little influence from his world, although he's ignorant of most of the customs through lack of interest; it's better to have something like this formalized than not. Not that he wants to make this into a formal courtship -- far from it, in fact, and he wouldn't really know how even if he did -- but he's groping for some kind of structure to figure out what's going on.
So, that answers "how." "Why" is a different matter. The question of attachment was answered in "On Kindness and Affection"; it would be easy enough to extend that into romance. (It's also true that, on rereading my canon, saying that Francescu is completely asexual is an overstatement. There's no onscreen relationship, obviously, but that doesn't *necessarily* mean that he'd never be interested in one. Regardless, pretty much asexual was how I started playing him, so.) I'm not going to do that, however; it's incomplete.
So, why Ken, and why romance? Basically, because of the way their friendship developed. Ken is seriously tactile, which I mentioned before; touch is tied up with sex in Unclean Legacy in general and I'd argue humanity as a whole. The only times people touch each other in Unclean Legacy are when they're lovers/married/courting, when they're fighting and, once, when Violet gives Sophie a hug and she shatters like glass. More importantly, Ken *cares* about people. (He's very like Violet, in that way; it was no accident that during Francescu's pastpost I dropped Ken into that role.) And it's very easy for Francescu to return caring; he's seen so little of it in canon that he cherishes what he gets. As a note, Francescu doesn't have a defined sexuality; he's basically attracted to one person right now as an outgrowth of a deep bond of friendship, and if he noticed anyone else it would take a similar depth of relationship, without regard for gender.
So, why so fast? Two weeks is a relatively short period to go from "asexual" to "relationship," or so it would seem. But when Gargamels fall for someone, they fall *hard* -- Manfred's relationship with Rachel lasted two days from meeting to sex, as far as I can tell. Even out of game, I've watched people who self-identify as asexual meet someone and start a relationship in days or weeks. (Okay, one person, but this person is also the only person who self-identifies as asexual I know IRL. Work with me, gentle readers.) So it's possible, and somewhat canonical at least.
It's worth noting that Francescu only has a vague understanding of what's happening. He knows enough to know that he cares about Ken deeply, and that he wants the thing that's growing between them, but otherwise, he's flying blind, so to speak. He'll deal with it as he's learned to do in camp, by talking and questioning and making sure that he's understood.
As always, I hope that this is helpful (and I hope it's coherent), and I welcome commentary and criticism.