* Ulysses Moore: The mysterious, eccentric, and brilliant inventor who lives in a giant treehouse filled with incredible inventions. He's a master of time travel and possesses a deep knowledge of history and science.
* Julia: A curious and adventurous girl who becomes Ulysses's apprentice and friend. She's intelligent, resourceful, and brave, often leading the group in tricky situations.
* Rick: Julia's best friend and a loyal companion. He's less interested in history and science than Julia, but he's always ready for an adventure. He's practical and dependable.
* Tim: A young boy who is initially skeptical of Ulysses and his inventions but becomes a close friend. He's loyal and helpful, often providing comic relief.
These four characters form the core group who embark on many time-traveling adventures, facing dangers and learning invaluable lessons along the way.