Aristotle was the founder of the Hystorians. He founded them because he thought that history had gone off course. Aristole was also the one who wrote the works that the trio had to save in Cave of Wonders.
Aristole first appears in the prologue of the book, where he discusses the future with his teacher, Plato, and agrees to mentor Alexander the Great. During the rest of the book, Aristole helps Dak, Sera, and Riq fix the Prime Break.