Originally he infiltrated Shinsengumi serving under Genzui KUSAKA, but Saeki neglected his duty to give information to Kusaka and placed himself in Shinsengumi for real, therefore he was killed by Kusaka.
もともと、久坂玄瑞の配下であり新撰組隊内に潜入していたが、佐伯の怠慢で情報を久坂に流さなくなり本格的に新撰組に身を置くようになったので久坂に殺された。