Cyrus was one of the bravest and strongest of the Knights of the Square Table in the Kingdom of Guardia in the years prior to 600 A.D. in Chrono Trigger. Glenn, who later became Frog, was his long-time friend. The first time the player sees Cyrus is in a couple of flashback sequences.
Frog flashes back to a time before he became a humanoid amphibian. He remembers how Cyrus protected him from bullies as a kid, and later when he enlisted in Guardia's army.
He also remembers when he and Cyrus left Guardia Castle in search of the Masamune with which to fight Magus' army. Along the way, they battled the Frog King. But later on in the journey, Cyrus was killed by Magus.
Crono's party heads to the Northern Ruins in 1000 A.D. but can't get into the Ruins due to Cyrus' ghost. So the party tried entering the Ruins in 600 A.D., but can't because the place is falling apart and is in need of repairs.
After the structure is repaired by the carpenter from Choras, the party finds Cyrus' tombstone and Frog pays his respects. Knowing Frog is safe, Cyrus' spirit is released and able to move on to the afterlife. But before he goes, he energizes the Masamune making it more powerful.