Spoiler!!!
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In their universe, the seme is actually what they call a "ghost", it's a special type of alpha, even different from the golden ones, where he is able to change a beta into an omega. But it's not just any other beta, the beta has to be a "diamond dust". In this case, yes, Yeehyeon is a diamond dust and he is secreting pheromones that only the ghost can detect.

Is he actually a beta??? Or is it like a mutation where he can just sense phermones but isnt it weird cause it seems to only be lius phermones genuinely wtf is going on with the uke im confused