Yamazaki Susumu

 was a Shinsengumi officer and spy, otherwise known as a .
He was a "rōnin" (masterless samurai) from Osaka and an expert in "Katori Ryu". In 1863, he officially joined the Shinsengumi and in 1864, Yamazaki and Shimada Kai were assigned by Kondō Isami to investigate the situation which led to the Ikedaya Jiken on July 8. However, there is a theory that Yamazaki's participation in the Ikedaya Jiken is merely fiction by Shimozawa Kan and Shiba Ryōtarō. 
His occupation whilst in Kyoto was as a doctor, and the Shinsengumi had originally used his pharmacy as their base before moving on to its later headquarters at Nishihonganji (See Yamanami Keisuke).
Yamazaki was said to be excellent in literary and military arts, having been taught by his uncle. His scholastic talent was necessary when discussing politics with a court noble. 
In 1865, he accompanied Kondo to Hiroshima. It is believed that Yamazaki did not return to Kyoto with Kondō until the next year in order to further investigate Chōshū's status.
Yamazaki studied medicine under Matsumoto Ryōjun. According to Matsumoto, Yamazaki was a gentle and taciturn man.
In 1867, Yamazaki became a hatamoto along with the rest of the Shinsengumi.
During the Battle of Toba–Fushimi in 1868, Yamazaki was severely injured and died on February 6. The exact location of his death is debatable, though it is believed he had drowned whilst escaping. He had Kondo and Hijikata Toshizō's full confidence and was one of the most faithful members of the Shinsengumi till the very end of his life.
Yamazaki in fiction.
Yamazaki is featured in "Peacemaker Kurogane" (anime/manga), Gintama (anime/manga), "Soshite, Haru no Tsuki", "Kaze Hikaru" (manga), "Getsumei Seiki" (manga), "Shinsengumi!" portrayed by Jun Hashimoto, "Bakumatsu Renka Shinsengumi" (video game series), "Destined to love" (otome game), and "Hakuouki: Shinsengumi Kitan" (anime/otome game). He is also depicted in the 1999 film "Gohatto". 
Even though the real Yamazaki was not a ninja, his anime, manga, and TV depictions tend to be a well-trained ninja. This includes Han'nya (a character loosely based on Yamazaki) from the manga "Rurouni Kenshin". Additionally, the character Yamazaki Sagaru in the manga "Gintama" is roughly based on Yamazaki. Haruka Takagi, the teacher of Isami from the anime series "Soar High! Isami" is also loosely based on Yamazaki.