Original name Koshima Shirou, Sagara came from a noble family. His father, Koshima Hyoma, was a wealthy moneylender. Sagara Souzou himself was a learned man in both militarism and national studies (something like Civics today), and he had almost 100 students when he was only 20!
He left his family to join the Ishin movement. His dream was the abolishment of the 4 class system, i.e. bushis (shi), farmers (no), artisans (ko) and merchants (shou). Very good friends with Saigo Takamori, and set up the first Sekihoutai under the latter's support and order in 1868, after the Bushin War. Members comprised mainly of peasant farmers, merchants, ronins and also some former Shinsengumi members (the Shinsengumi broke up after its defeat in the Boshin War). Sagara served as Captain of the First Patrol, and following is the list of the other captains:
Main Patrol: Ayanokouji Toshizane, Shigenoi Mikihisa (both former
imperial court officials)
1st Patrol : Sagara Souzou
2nd Patrol : Suzuki Mikisaburou (yep, the former Shinsengumi member)
3rd Patrol : Yukawa Rensaburo.
Sagara used to create disturbances, setting fires in Edo to disrupt the Bakufu's adminstrations, also under Saigo's orders, who instructed the Sekihoutai to go ahead of the Reformation army. That is, to make sure of the situation and reactions of the clans along the way before the Ishin army reached that particular place. Also, the Sekihoutai acted as a kind of side-kick to the Ishin army, announcing that under the new government, the annual tax would be reduced to half. Actually this was the government's political ploy to gain more support among farmers. The Ishin government really did not have that much money to sustain the country. Therefore, once the Sekihoutai was of no more use to them, they announced the troupe to be 'false armies' on 10/2/1868 and attacked them, which comprised mostly of peasant farmers and merchants. The Sekihoutai was finally defeated in Shitasuwa (where Sanosuke first met Anji and was reunited with Sagara Taichou's spirit), and Sagara was captured on 2/3 the same year, and decapitated the second day, 3/3, at the age of 30. Like it or not, the tragedy of the Sekihotai is a dark episode of betrayal in the Ishin Movement. Cruel, but more often than not, that's history for you.
(I still feel thoroughly moved whenever I read parts where Sagara Taichou appeared, either in Sanosuke's reminiscences or his ghost... From the bottom of my heart, I salute him!)