For anyone not familiar with the characters, Bertie's Aunt Agatha does not actually have cloven hooves for feet. She does have a very overbearing personality and an amazing ability to terrorize poor Bertie. She's often referred to by Bertie and his cousins, Claude and Eustace, as "the nephew-crusher". Her main goal in life seems to be seeing Bertie married off to a suitable woman of proper social standing, something Bertie is very against.
According to Bertie, "I dare say there are fellows in the world -- men of blood and iron, don't you know, and all that sort of thing -- whom she couldn't intimidate; but if you're a chappie like me, fond of a quiet life, you simply curl into a ball when you see her coming, and hope for the best. My experience is that when Aunt Agatha wants you to do a thing you do it, or else you find yourself wondering why those fellows in the olden days made such a fuss when they had trouble with the Spanish Inquisition." [Wodehouse - "Extricating Young Gussie"]
Agatha became Lady Worplesdon later in the series. After the death of her first husband, Spenser Gregson, she married Percy Craye, the then Lord Worplesdon and Lady Florence Craye's father. Since I mean this all to be taking place after the books, I decided to go with her as Lady Worplesdon instead of Mrs. Gregson.