Mr Wentworth, his wife, and Brian Wentworth are all witches. Mrs Wentworth is not actually dead; she went into hiding after being exposed as a witch, and Mr Wentworth still hopes to be reunited with her some day.
Miss Cadwallader knows that Mr Wentworth is a witch and she blackmails him, taking most of his salary. This is the reason for his apparent miserliness.
Chrestomanci reveals that Charles Morgan's magic is near enchanter-class, especially after examining the spell that Charles used to make Simon?'s every word become true.
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