A large part of starting a fic is world building. And finding and accepting one piece of canon can change the face of your entire world, much to your chagrin. So this world is going to be more complicated than I originally thought. And this fic is going to be a bit longer than the one shot I was originally predicting. Of course Urahara would be the one to completely upset my plans.
It all comes down to perceptions. Youkai are easy to see, if you know where to look and can accept what you’re looking at. Hollows are trickier. A character can only see a hollow if they possess spiritual power or are exposed to a large source of it for a period of time. The Keikain Clan uses spiritual power to summon shikigami, create barriers, and for other types of onmyojutsu. They should all hypothetically be able to see ghosts.
But youkai use Osore, or Fear, to power their abilities. So they probably can’t see a hollow, but they would be able to figure out pretty quickly if something invisible was preying on them, and would usually be able to escape or even hurt it. Not to mention . . . Hollows feed on human souls. And youkai aren’t human, and thus don’t have human souls. Conveniently enough, Rihan and Rikuo are the ones most likely to be able to see hollows. They both have human blood, and Yohime canonically has spiritual power in the form of her healing gift.