Who is Jack Favell?

Jack Favell was Rebecca's cousin and lover. The narrator first meets Favell at Manderley during Maxim's absence and is put off by his bold manner and suggestive remarks. After Rebecca's body is discovered, Favell contends that Maxim murdered her and attempts to prove that they were planning to run away together. He brings in Ben as a witness, displays a note from Rebecca, and prompts the trip to London to interview Dr. Baker. After Dr. Baker seems to confirm the coroner's findings, Favell returns to Manderley and presumably sets the estate on fire with Mrs. Danvers.

Jack Favell is Rebecca's cousin

Jack was Rebecca's cousin. I don't think the term 'lover' quite describes their relationship. He may have thought Rebecca was going to leave Max and marry him but I doubt Rebecca considered doing that for a second. Why leave Max's money, social standing and the house she adored for someone with nothing and no prospects, she wasn't the type to go all soppy about love. Max even says he thought her incapable of love. Mrs. Danvers attests that they had an on-going sexual thing for years but that Rebecca wasn't in love with him or Max or any man and that she was the only one Rebecca loved. I doubt this also, more likely Rebecca led her to believe this just as she led Favell to believe it, another of her jokes. Jack called Mrs. Danvers to tell her what Dr. Baker said. It is highly unlikely that he jumped into his car after the meeting with Baker and made the six hour drive to Manderley to set fire to the house, more in character for him to dive into a bottle at his flat and brood. Mrs. Danvers set Manderley on fire to prevent Max and the new wife living happily ever after in her darling's beautiful house. Jack was undoubtedly the bounder and cad Max, Bea and the new wife describe but he was nobody's fool and was certainly correct in his opinion that Max killed Rebecca. He possessed the sly cunning and charm a lot of bounders and cads have and I always liked the way he said 'fare you well' upon departing.