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Omegaverse is a versatile genre which depends upon the author wishes. This omegaverse is different from others because the author created new rules (although I don't think they follow their own rules strictly). Basically in a typical omegaverse it's the alpha which bites the neck of the omega and marks them. But in this, the omega bite the ear lobe (I think) to permanently mark the alpha. An alpha can mark an omega but it would be a temporary mark. The omega can have several marks from different alphas at the same time.
Only an omega can make a permanent mark and it would last a lifetime on an alpha. The alpha won't be able to smell pheromones other than from the omega who marked them.
(〜 ̄△ ̄)〜
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It was mentioned in the 'Kiss me Liar' novel that there are many cases of an alpha killing their omega partner who permanently marked them once their love fades. That's why Keith mentioned that he will kill the omega who marked him since the mark is permanent. Yeonwoo was hiding that he marked Keith because he was afraid of getting killed (and also because he wanted a revenge on Keith or something I don't remember ╮( ̄▽ ̄)╭).
did they change it??? wasn’t the omega the one who marks the alpha first why is it the other way around???