They are the outcasts of the different clans of creatures that have formed their own clan.  Being half human they not accepted nor respected by the full blood creatures.  Even though they are half human they have the same powers as the full bloods do but there as minor changes when it comes to their creature form.  The half-mermaids have multi-coloured tails, the half fairies wings fold fully into their backs and appear as a tattoo, the half-fall angels have more humanity in them but are much more deadly than full blood fallen angels and the half-demons have no differences from the full blood demons.

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Gunwoo - half mermaid

Insoo - half fairy

Seyong - half fallen angel

JunQ - half fallen angel

Chaejin - half demon