Margalo is a little brown bird who becomes Stuart's first friend his own size, known for her spirited tales of beautiful places, inspiring Stuart to embark on a quest of his own.

Snowbell is the family's cat, who acts as an antagonist by often resenting and teasing Stuart, highlighting the tensions between his instincts and living with a family that includes a mouse.

Stuart's parents and older brother, who cherish his uniqueness and make adjustments at home to accommodate him; George, in particular, often starts projects intending to help Stuart but struggles to complete them.

A dentist and Stuart's friend, Doctor Carey owns the miniature schooner, the Wasp, and symbolically enables Stuart's adventurous spirit by providing him with opportunities and miniature modes of transportation.