William Cavendish, 3rd Earl of Devonshire, was a teenager under the tutelage of Thomas Hobbes when the time-displaced town of Grantville arrived in the past. He went on a Grand Tour of the European continent with his tutor, officially as part of a rite of passage for English nobility. In reality, Hobbes had been tasked by the Earl's mother to travel to Grantville to discover the future of the Cavendish family.
William was a very curious student who often got himself into trouble. The things he did included planning to lower himself into the volcano Mount Vesuvius and soliciting a courtesan (for a game of chess, much to Hobbes' relief). Cavendish later learned that, in the OTL, Athanasius Kircher had been lowered into Vesuvius. Hobbes privately concluded that William wouldn't excel in literature after hearing and reading William's travel journal. Hobbes was impressed with William's skills as a mountaineer while they crossing the European Alps. Upon arriving in the New United States, William was excited by American innovations.
While in Grantville, William befriended the "Barbie Consortium" when he witnessed them playing the modern version of tennis. He also became infatuated with Judith Wendell, a fact which initially panicked Hobbes, who feared the wrath of Lady Cavendish when she learned of her son's "foreign romantic entanglements". However, when Hobbes learned that Judith's father, Fletcher Wendell, was the NUS Secretary of the Treasury, and thus, similar to a nobleman, he concluded that Judith was a suitable match for William. William later insisted on Hobbes bringing the Barbie Consortium on their trip to Magdeburg, though his tutor was reluctant to do so.
In July 1633, William and Hobbes traveled to Magdeburg to meet Gustavus Adolphus. Initially, the King of Sweden was uninterested in seeing them, and gave them fifteen minutes for the meeting; until William raised the issue of tennis, which was Gustavus' favorite sport.
On September, William was rudely brushed off by some of his up-timer friends, specifically the Mason family, Derrick and Kelsey Mason. He soon learned from Hobbes that England had become a member of the League of Ostend, which had just defeated the Dutch fleet, and that the English government had imprisoned the Grantville delegation in the Tower of London. As the members of the delegation were popular in Grantville, and England was known to be a member of the League, William learned that he and Hobbes had became persona non grata in the town.
Also, the two were concerned because the Privy Council could interpret remaining in Grantville as treason, as it could be considered "enemy territory". Sadly, William went with Hobbes to Hamburg to catch a ship back to England. Before leaving, William had a last meeting with Judy, who apologized to him for the rude response he'd received from the Masons, as the Treaty of Ostend was not his fault, and he should not be held responsible based on his nationality. William understood, and promised to keep in touch with Judy through mail.
At his departure in Hamburg, William learned from Hobbes that he had decided to stay in Grantville to continue his research on the Cavendish family history, and so that he could send word back home of any critical new developments. So, William was tasked with telling this news to his mother and acting as a relay to Hobbes, without actually using Hobbes's name. After returning to England, William sent letters to Judy and Hobbes, which the latter received because the address was a drawing of the tiger Hobbes from Calvin and Hobbes, wearing academic garb. William had seen The Complete Calvin and Hobbes during his stay in Grantville, and decided that it was something that only the Grantvillers were likely to associate with Hobbes.