Eddie was Frankie lovers after his affair with Columbia. When he started his project, Eddie took up with Columbia (who I think was human as well) That went on until Frankie needed half a brain for Rocky. The rest is…well, left overs.

Frank used Eddies brain to put in Rocky…ergo the scar on his forehead. he was once both Frank and Columbia’s lover and I think he just ended up at franks castle by happenstance.

Eddie was Frank’s former lover, but not a creation of his. He was in fact Dr Scott’s nephew which is why he turns up later on in the film looking for him.