Kato is a full-blooded saiyan born on Earth hidden from society. He and his brother Drey were trained by their father until he was killed by another saiyan. He fought the entire the Z-Fighters because he thought that they killed his father. He beat Krillin, Tien, Yamcha, Pan, Uub, Goten, Trunks, and Gohan back to back. But when he fought Vegeta he was beaten. When Vegeta was about to kill him, Goku intervened and said that Kato was saiyan. Everyone was shocked by the news. After that they found the saiyan who had killed his and Drey's father, it was Broly. He had somehow got free from hell and came to Earth to kill people. The Saiyans fought for a long time and Broly was winning. Broly then went after Drey and beat him into the ground. Kato was furious at what he was seeing, on top of that he couldn't even help his brother. Mad still he charged Broly but was hit back. Rage surged through and then he transformed into a Super Saiyan 6. Kato launched himself to Broly and utterly destroyed him. Broly was caught off guard by his power. Kato charged up his strongest attack and threw at him. Broly was engulfed by the blast. The blast then imploded than exploded killing Broly. After that, they treated Drey's wounds and then the 2 brothers joined the Z-Fighters to fight off all evil that threatened the universe.
Kato has the personality of Goku, he is light-hearted and takes it easy, but when angered he becomes a force to be reckoned with. He can be very protective of his brother. Sometimes he does not care what he does, only to make sure Drey is safe.
In his base form he could take on a full powered Gohan, but with Vegeta, he would have to be at least a super saiyan 2. With Goku, he would have to be at full power in SSJ6. He is the 3rd strongest member of the Z-Fighters.