In order of appearance:
- Enoch Root – an elusive and mysterious alchemist who first appears at the beginning of the book and recurs throughout often in the company of Alchemists such as Newton and Locke.
- Daniel Waterhouse – son of prominent Puritan Drake Waterhouse, roommate of Isaac Newton, friend of Gottfried Leibniz, and prominent member of the Royal Society. Waterhouse is both a savant and a strict Puritan. As Quicksilver progresses he becomes more and more involved in the inner workings of British politics.
- "Half-Cocked" Jack Shaftoe – an English vagabond, known as "The King of the Vagabonds", who rescues Eliza and becomes the enemy of the Duke d'Arcachon.
- Eliza – a former harem slave who becomes a French countess, investor, and spy for William of Orange and Gottfried Leibniz. She originally became a slave when she and her mother were kidnapped from their homeland of Qwghlm by a European pirate with breath that smelled of rotten fish.
- Robert Boyle, Irish natural philosopher
- Caroline of Ansbach, an inquisitive child who loses her mother to smallpox
- John Churchill, former employer of Jack and a prominent British politician
- William Curtius, German Fellow of the Royal Society, and diplomat for the House of Stuart.
- Nicolas Fatio de Duillier
- Judge Jeffreys, Lord Chancellor of England
- Robert Hooke, English natural philosopher and biologist
- Christiaan Huygens, continental natural philosopher
- Gottfried Leibniz
- Louis XIV, King of France
- Isaac Newton
- Henry Oldenburg, founding member and secretary of the Royal Society
- Bonaventure Rossignol, a French cryptologist
- James Scott, 1st Duke of Monmouth
- James Stuart, as the Duke of York and as James II, King of England
- Edward Teach, aka Blackbeard
- John Wilkins, Bishop of Chester, founding member of the Royal Society, and advocate of religious tolerance in Britain
- William III of England, as William, Prince of Orange