Lucy Doyle
Lucy Doyle was Agatha Witt's favourite apprentice, and still one of her favourite people. She was Gabriel Edgarton's apprentice mistress herself, bringing a degree of order and necessary structure to his life. She is the person you want doing the testing when you're evaluating tiny particles of sand.
She is - unlike most of the Penelopes - referred to reliably by both of her names, suggesting that there's another Doyle in regular contact in those circles. Her father is a solicitor, including for Giles Lefton.
The Hare and the Oak
Lucy leads the group of Guards investigating what's going on at Baddock Hall in the later chapters of the book in The Hare and the Oak.
The Magic of Four
Lucy makes an appearance late in the book, as someone who's known Avigail since before she was born.
(Click on the title above to learn more about Doyle's role in that book.)