Tyson is a Cyclops, and Percy's half brother. He was born of Poseidon and a nature spirit. As a young child he was left in the streets to fend for himself.
After being continuously bullied, he prayed to Poseidon for a brother, someone who would take care of him. So, Poseidon sent him Percy, who only knew of Tyson as a friend until they went back to Camp Half-Blood and were attacked by Hephaestus' bulls when Tyson revealed himself immune to fire and confirmed Annabeth's suspicions of Tyson's species.
During the aftermath of the Second Olympian War he became general of the Cyclops army, his weapon being a "stick" (club).