Formally the leader of the 3rd squad in Shinsengumi, he and Kenshin was bitterly rivals. After the Meiji restoration, he worked as an assistant police officer and came to the dojo one day to find and challange Kenshin to a duel. Saitoh disguised himself as a saleman and entered Kamiya Dojo. Sanosuke saw though his disguise and attempted to defeat Saitoh. Saitoh defeated him easily and left Sanosuke injured. A few days later, he came back again under a false alias Fujita Goro. Finally after ten long years, he finally meets Kenshin. They both engage in a hard fought battle and were on the verge of death. However, just when the battle was up for grabs, Kawaji, the chief of police stopped the fight. Apparently, Saitoh's main purpose was to test Kenshin abilities and to ascertain whether he was still worthy to lead an attack on a deadly terrorist, Shishio. Saitoh was working for the Meiji government as a secret service officer.

It is ironic that Saitoh now works for the very same government that he opposed. His most famed beliefs are "Aku Soku San" or "Sin Swift Slay." The english translation is kill evil instantly. Because of these principles and beliefs, Saitoh willingly work to preserve peace in the Meiji government. Saitoh special attack is known as "Gatotsu" or "the Left Horizontal Thrust." This special strike of the Shinsengumi was invented by the deputy leader of the Shinsengumi, Hijikata Toshizo. His ougi is known as "Gatotsu Zero-Tsuki" or "Fang Stab Zero Skill." This is a very powerful move because this move allows Saitoh to attack at point blank range.