Arthur Stanley is the son of Prudence and Edward Stanley, brother of Guy and cousin of Phryne Fisher. He is shown to have an undetermined mental and physical condition (potentially regressed infancy) which limits his faculties, requiring him to remain in the care of his mother.
As of Death and Hysteria, Arthur was revealed to have passed away recently in his sleep.
Prior to the series, little is revealed of Arthur's history and it is unknown if his mental condition was present in his youth however it is revealed that he was present at the circus when Murdoch Foyle kidnapped Jane Fisher and that he later identified Foyle in a police line up which assisted in his initial conviction and imprisonment.
If Arthurs condition was not present in youth then it may be assumed that his condition arose from Shell Shock if he served during the First World War (although Guy's outburst in Murder in the Dark suggests that his condition was present from youth).
Arthur was revealed to be present when Murdoch Foyle appeared at the Stanley Estate prior to his brothers engagement party where he murdered Marigold, Foyle then gave Arthur a toffee apple which he was enjoying as Marigold's corpse floated in the pool near him where they were discovered by Prudence (Arthur believed Marigold was sleeping).
Arthur is later seen during Jack and Phryne's inital investigations into Marigold's murder, where Phryne gives him a present of large gobstoppers which he retunrs to his room to enjoy (at Prudence's insistance) and it is assumed that he was questioned by Jack as part of his investigation.
He was shown later to be spending time with Jane Ross who was reading Hansel and Gretal with him when Phryne appeared with another present for him, a Hansel outfit, to which he happily exclaimed that Marigold would be his Gretal. Phryne then told him that Marigold would not be there, he becomes saddened and blames himself, thinking Marigold has left because she was upset with him, Phryne comforts him before Prudence intervenes and takes Phryne asside to discuss revealing Marigold's death to Arthur.
During the brief moments that Phryne and Prudence talk, Jane begins teaching how to write her name. Upon realising that Jane's name was the same as his cousin (Jane Fisher), Arthur flies into a rage, throwing his possessions around and crying out for his cousin (much to Jane's distress), as he calms down he asks if "The Woodcutter" who had taken Jane, had also taken Marigold. Prudence reveals that Marigold was gone like Jane to which he responds that he wishes he could take revenge on the "Woodcutter", Prudence quickly tries to calm any further violent ideas.
Later that evening, during the engagement party, Arthur appears to be confined to his room and is in bed when Phryne comes up with a tray of sweets for him, as she leaves, he throws off the covers to reveal himself to be wearing a wolf costume (the opposite of Jane's Red Riding Hood costume) and he disappears into the party when Phryne and Jack try to find him.
He is somehow accosted by Guy who furiously wrestles with him, claiming that he should be locked up and that through all of his life Arthur had always taken presesence with his mother. Arthur then fights back, attempting to strangle guy but is stopped by Jack.
As he is questioned by Jack, Phryne realises that Arthur has dropped another toffee apple to which she desperately questions him and he reveals that "The Woodcutter" had given him the treat in exchange for his costume leading Phryne to realise Foyle was alive (which is later confirmed by Dot and Hugh), Phryne and company then leave and it is assumed that Arthur was left in his mother's care.
Sometime after the events of Murder in the Dark, Prudence employed the help of Dr Hayden Samuels, a natural healer and Psychologist, to work with Arthur (who's condition may have been deteriorating). It is revealed that Dr Samuels utilised a form of musical therapy with Arthur (in particular the song "There's a Long Long Trail A Winding" as revealed by Cecil Yates) which appeared to produce a positive result from Arthur.