The same age as
Okita Sōji and another member named
Tōdō Heisuke, the three shared the distinction of being the youngest in
Kondō Isami's group and being among its most gifted swordsmen. As a member of the
Shinsengumi, Saitō was said to be an introvert and a mysterious person; a common description of his personality says he "was not a man predisposed to small talk". Saitō was an unusually tall man at . He was also noted to be very dignified, especially in his later years. He always made sure that his obi was tied properly and when he walked he was careful not to drag his feet. At rest he always sat in the formal position, called seiza, and he would remain very alert so that he could react instantly to any situations that might occur. He was, however, known to be very intimidating when he wanted to be. Along with his duties as Captain of the Third Squad in the Shinsengumi, he was also responsible for weeding out any potential spies within the Shinsengumi ranks. Members had to constantly be mindful of what they said around him. His original position within the Shinsengumi was . His duties included being a
kenjutsu instructor. During the
Ikedaya incident on July 8, 1864, Saitō was with
Hijikata Toshizō's group that arrived later at the Ikedaya Inn. On August 20, 1864, Saitō and the rest of the Shinsengumi took part in the
Kinmon incident against the
Chōshū rebels. In the reorganization of the ranks in November 1864, he was first assigned as the fourth unit's captain and would later receive an award from the shogunate for his part in the Kinmon incident. At the Shinsengumi new headquarters at
Nishi Hongan-ji in April 1865, he was assigned as the third unit's captain. Saitō was considered to be on the same level of swordsmanship as the first troop captain
Okita Sōji and the second troop captain
Nagakura Shinpachi. In fact, it is rumoured that Okita feared his sword skill. Despite prior connections to Aizu, his descendants dispute that he served as a spy. His controversial reputation comes from accounts that he executed several corrupt members of the Shinsengumi; however, rumors vary as to his role in the deaths of
Tani Sanjūrō in 1866 and
Takeda Kanryūsai in 1867. His role as an internal spy for the Shinsengumi is also questionable; one common example being that he is said to have been instructed to join
Itō Kashitarō's splinter group
Goryō Eji Kōdai-ji faction, to spy on them, which eventually led to the
Aburanokōji incident on December 13, 1867. However, this is disputed by Abe Jūrō, who did not believe he was a spy. It is probable that he also monitored other intelligence and enemy activity. Together with the rest of the Shinsengumi, he became a
hatamoto in 1867. Later in late December 1867, Saitō and a group of six Shinsengumi members were assigned to protect
Miura Kyūtarō, who was one of the major suspects of the assassination of
Sakamoto Ryōma. On January 1, 1868, they fought against sixteen assassins who were trying to kill Miura in revenge at the Tenmaya Inn on what was known as the
Tenmaya incident. After the outbreak of the
Boshin War from January 27, 1868 onwards, Saitō, under the name , took part in Shinsengumi's fight during the
Battle of Toba–Fushimi and the
Battle of Kōshū-Katsunuma, before withdrawing with the surviving members to Edo and later to the
Aizu domain. Due to Hijikata being incapacitated as a result of the injuries sustained at the
Battle of Utsunomiya Castle, Saitō became the commander of the
Aizu Shinsengumi around May 26, 1868 and continued on into the
Battle of Shirakawa. After the
Battle of Bonari Pass, when Hijikata decided to retreat from Aizu, Saitō and a small group of 20 members parted with Hijikata and rest of the surviving Shinsengumi and continued to fight alongside the Aizu army against the imperial army until the very end of the
Battle of Aizu. This parting account was recorded in
Kuwana retainer Taniguchi Shirōbei's diary, where it was recorded as an occurrence also involving
Ōtori Keisuke, whom Hijikata requested to take command of the Shinsengumi; thus the said confrontation was not with Hijikata. However, questions regarding this parting remain, especially considering the conflicting dates. Saitō, along with the few remaining men of the Shinsengumi who went with him, fought against the imperial army at Nyorai-dō (a small temple near
Aizuwakamatsu Castle), where they were severely outnumbered. It was at the Battle of Nyorai-dō that Saitō was thought to have been killed in action; however, he managed to get back to Aizu lines and joined the Aizu domain's military as a member of the
Suzakutai. After
Aizuwakamatsu Castle fell, Saitō and the five surviving members joined a group of former Aizu retainers who traveled southwest to the
Takada Domain in
Echigo Province, where they were held as
prisoners of war. In the records listing the Aizu men detained in Takada, Saitō is on record as Ichinose Denpachi. ==Meiji Restoration==