Otherkin is a group of people, some practitioners, some not that have a spiritual connection to something. usually people feel the bond with Vamps and Wolves but i saw some that were Fay and there is like a million more. there are different degrees some people just feel the contentedness others feel a real contentedness or that they really have the spirit of something in them.
when people aren't born as true kin (insert type here) they try to build a contentedness with the (insert whatever you want to be close to here). I heard of people invoking demons to become considered demonkin and wolves to be wolfkin.
you do realize after invoking/evoking and drawing it to you, you will show some of the non physical attributes of the demons your trying to bond with and it could make your life stink though right?
i think you should look into it and ask questions before you start doing anything but it's your life and you can do with it what you want.
You can't become a otherkin, The actual lore of Otherkins are that they are born the way they are, with a soul of a nonhuman within a human body. I don't believe theres a way to turn your soul nonhuman.