She was the daughter of Queen Charlotte and King George III and the second wife of King Frederick I of Württemberg. They had one stillborn child. Her Wikipedia article is here.
Charlotte Mathilde keeps up with the Bonapartes by wearing a cherusque and jeweled waist band matching her tiara in this portrait.
Keywords: Charlotte Mathilde, Welf family, Württemberg family, Princess, Queen, curly coiffure, tiara, square neckline, modesty piece, lace, neckline ruff, long tight upper puffed sleeves, back-flared cuffs, jeweled waist band, Empire waistline, close skirt