Cleveland Booker's young nephew on Kwejian, whose death in the planet's destruction shaped Book's grief throughout Discovery's fourth season.
Leto is the nephew of Cleveland Booker and son of Book's brother Kyheem. In Kobayashi Maru, Book returns to Kwejian to take part in Leto's coming-of-age ceremony. Book flies off-world to investigate strange bird activity just before a gravitational anomaly destroys the entire planet, and Leto is lost along with the rest of his family.
In Anomaly, Book is numb with grief and haunted by hallucinations of the boy. In Choose to Live, Ni'Var President T'Rina helps him relive memories of Leto, which brings him some comfort.
The loss drives Book's later alliance with Ruon Tarka against the Dark Matter Anomaly. In Life, Itself, the son of Michael Burnham and Book, a Starfleet captain seen years later, also carries the name Leto.