Miss Agnes Adderstone is a bone skinny, mean woman who has false teeth. She is believed to have reddish-grey hair and black eyes.

Miss Adderstone is the headmistress of Hardwick House who dislikes children, especially babies. She hates Molly Moon especially out of all the orphanage's children. Later on in the book, Molly hypnotises her to love planes and flying.

Agnes was absent during Book 2, but returned in Book 3 when Molly travelled back in time to return her past selves to their proper times. She was instructed to fart and burp in company, as well as to be kind to the children under her care when she was very drunk.

Before hypnotised by Molly, Petula the pug belonged to Miss Adderstone, who fed Petula chocolate biscuits that caused stomach aches.

Adderstone favors Hazel, because it is learned that she looked like her when she was dropped off on the doorstep of the Hardwick House.