For harem reasons, probably. According Kuusetsu, what she did is basically make Yuuki a receptacle for the gods and whatever they truly are. Yuuki did touch Fukuma yes, but they also kissed briefly too. Kuusetsu did sorta allude to the physical contact being enough but, i have a feeling it had...
Yeah i didnt go that deep, just the general meaning for each name Rai and Ren. My bad on that ^^;; though that said its an interesting choice given that Rai is typically masculine and while Ren is more gender neutral, it tends to be used for women quite often when...
I mean, its never been stated specifically that Rairen is either male or female, and a few very manly looking characters have been women. Not to mention that Rairen can be translated a few different ways, but the most appropriate would be thunder lotus blossom...Now i can just picture Rairen...
Im guessing it either has something to do with Kuusetsu putting some of her power into him in the first place, or it happened when Fukuma kissed Yuuki as Bell. Kuusetsu could sense her through the momentary contact.