Ten years ago, Sanosuke Sagara was a junior member of the Sekihoutai, an unofficial division of the Meiji Government Army lead by Sozo Sagara, whom Sanosuke idolized (to the point of adopting his last name). However, when the Meiji Government realized that it couldn't keep all of its promises, it pinned all the blame on the Sekihoutai, which was subsequently disbanded. Sozo Sagara was beheaded for a crime he did not commit, which is something Sanosuke will never forgive the Meiji Government for. However, a fight with Kenshin showed him that not all of the people who created the Meiji were as bad as he thought. He and Kenshin are the best of friends now, with similar ideologies if executed through different methods...

Sano is the tough guy of the series, with indomitable strength and an attitude characterized by the Japanese character aku, or "bad," on his back. He takes pride in the fact that he's undefeatable, and will repeatedly challenge anyone who does defeat him until he wins (with the exception of Kenshin). Sano seems to live by the one rule of "defy authority," and although he prefers the high route of government takedown (enlightening the people as to the truth), a good fight is never above him. (A bad fight, on the other hand...)