Kin technically means any magical being of mixed ancestry. Colloquially, however, it generally refers to “human” wizards.
Humans cannot use magic, but Kin descended from humans (and other things) can, giving rise to the legends of human magic-users in all cultures. To that end, Kin have a wild variety of powers depending on whatever they descended from. Unless they’re part of a group or family who’ve been stabilized, genetically, for a while, their abilities are unpredictable.
Most Kin appear to be human, but quite a few others have broken off into their own haplogroups. Among the Kin, there are nine powerful families generally in charge – hence the nine leaves on their branch.
Kin specialize in use of iron in their magic, as well as potions and chemistry, which they need because their own power tends to be diffuse and hard to focus without an aid. They do their best to live among humankind, trying to blend in, grieving when they have to move on because their friends are aging and they are not. It’s a challenging thing to be Kin; not quite other, not quite human, they never fully fit in outside of other Kin.