I have never heard of a proper name for a half human/ half fairy and I read a lot of fairy books. Some authours prefer to make up their own terms for somthing like this. This is just suggestion for if 당신 can't find the proper term. I hope this gives 당신 some ideas even if it doesn't sound very helpful.
Basically what I was going to say is the suggestion above. I write some 판타지 and stuff, and I just make something up. 당신 could use crossbreed 또는 something.
