Since I received a lot of messages like this I’ll publish this reply, sorry for anyone who isn’t interested. So here’s my headcanon for Farbauti (nothing special but ok).
She was an extremely powerful sorceress and shapeshifter (I imagine her to be not fully Jotun maybe Vanir) took prisoner by Laufey who made her his concubine NOT his queen or consort and locked her up with magical invisible chains. She gave birth to many children but only Loki survived. That makes Loki Laufey’s bastard son and first born and only heir? However she died after giving birth to him, I like to think that her last words (and first to her son) were “kill Laufey and rule them all” or something like that.
I want to specify that in Norse Mythology Farbauti is Laufey’s HUSBAND and Loki’s FATHER and I like that interpretation too. My headcanon can be applied to both Laufey or Farbauti, as you like.