William Frederickson comes from Westphalia. Nicknamed 'Sweet William' by his men, Frederickson has suffered a serious facial wound which has destroyed his left eye, broken his jaw, knocked out several teeth and caused nerve damage giving him an uncontrollable and permanently smug look. When fighting he takes out his false teeth (taken from dead French soldiers) and removes his wig and eye patch, to terrifying effect. He is however a cultured and sensitive man, with an informed knowledge of European architecture. He is of German and English descent and speaks both languages fluently, as well as competent French in Sharpe's Siege and Sharpe's Revenge and passable Italian Sharpe's Revenge.
He is a close and loyal ally of Sharpe until the fallout over the affections of Lucille Castineau, who rejects Frederickson's proposal of marriage before taking Sharpe as a lover, as she is carrying his child Sharpe's Revenge. This makes Frederickson angry at Sharpe, and ends their friendship, so he ends up leaving Sharpe. A discussion between Sharpe and Harper in the lead-up to the Battle of Waterloo indicates that Sweet William has been posted to Canada to serve in the War of 1812.
In the Sharpe adaptation, Frederickson was played by Philip Whitchurch. Instead of leaving Sharpe, he forgives him at the end of the episode.