Belthasar is the Guru of Reason from Chrono Trigger.
He once lived in the Kingdom of Zeal in 12,000 B.C., but an accident threw him through time and into 2300 A.D. He also explains that the entity called Lavos is constantly consuming the energy from the planet, slowly killing it and all living things on it.
Because he was growing old and frail, he downloaded his memory into the Nu. He then allows Crono and his friends access to his final invention, what he calls the "Wings of Time", the Epoch.
In a flashback sequence, it's revealed that Belthasar was transported from 12,000 B.C. to 2300 A.D. due to Lavos' power.
After Crono's death and Magus joins the party, they return to Keeper's Dome a couple times to speak with Belthasar about reviving Crono.