Lady Marcham is a non-canonical character, created for the 2022 miniseries Why Didn't They Ask Evans?, an adaptation of the Agatha Christie novel of the same name. She is Frankie's rather temperamental mother who is probably an artist – the only time she appears, she is in a dishevelled state and there is a paint smudge on her cheek.
She first reprimands Frankie for ruining her dress (which has been covered in Bobby's vomit due to poisoning) and for stealing from her wardrobe all the time, then gets upset at her husband not remembering the dress she wore for their anniversary. However, it is evident that she loves her family very much.
She provides the young sleuths with a vital clue when she claims that the parlourmaid is the only one who knows what is going on in the house. Frankie later connects it to the witnesses of Savage's will, concluding that only the maid could have properly known what he looked like.