Calvin, the Tiny Hero, is a Nipsie who has been enlarged by magic, making him a giant amongst his own kind, though he stands only three inches tall.
Calvin goes to Seth Sorenson, initially appearing a mouse until Seth drinks the milk to see Calvin's true form, and Calvin explains that Seth, as the slayer of Graulas, is a part of a nipsie prophecy to undo a magic curse that was cast upon the nipsies long ago, making them less than what they originally were.
Calvin was selected by the leaders of his people to serve Seth and was magically empowered to make him more able for the job. Seth accepts Calvin's service but asks him to be his friend rather than his servant. Another reason Calvin wishes to go adventuring is to find his beloved, Serena, another nipsie who left Fablehaven long ago to learn of the curse on the nipsies.
Calvin accompanies Seth and his sister Kendra to Wyrmroost after the siblings have accepted Agad's request to become the new caretakers of the sanctuary, and Seth introduces Calvin to his sister, grandparents, and Marat, who suggests Calvin could be a useful spy as long as the staff of Blackwell Keep don't become aware of him.