In the short stories Poirot and the Regatta Mystery and the Parker Pyne short story The Regatta Mystery, Mrs Janet Rustington is a young writer and member of the yachting party hosted by Isaac Pointz at Dartmouth. She attended the dinner at the Royal George Hotel during which the Pointz's diamond, the Morning Star went missing.
Her host, Isaac Pointz found her an attractive person, clever but charming. She wrote what he considered "high brow stuff" but didn't "ram her writing down your throat". Nonetheless, she was also quite aloof. This was partly explained by her past experiences. She had apparently married young but the marriage had ended in disaster after less than a year which made her wary of trusting anybody. The nature of the disaster was not explained in the text except that her husband had been a "down and out rotter".
This is probably one of the earlier ues of the name Janet for a Christie character. This would be a common name in future stories. Often "Janet" is used for characters in domestic service, and then they would generally not be given a surname.