was a Japanese samurai of the late Edo period. A highly skilled swordsman, he was one of the four most notable assassins of the Bakumatsu period. Gensai's high-speed sword discipline allowed him to assassinate targets in broad daylight.
He was born in Kumamoto, Higo Province, Japan in 1834 as the second son to , a retainer of the daimyà  of the Kumamoto Domain and his wife . Because Genjiro's older brother Hanzaemon was chosen as the family's heir, at age 11 he was given in adoption to , another Kumamoto retainer and was renamed to Kawakami Gensai.
He then entered the domain's school, the Jishà «kan (æÂÂç¿Â館), and followed its academic and martial courses of study. At age 16 he was called to serve in the Kumamoto castle town as a menial in charge of cleaning (Osà Âji-bà Âzu ãÂÂæÂÂé¤åÂÂ主).
Although this was a low-level position, Gensai devoted himself wholeheartedly to it, using his free time to polish his martial and literary skills, as well as learn sadà  (tea ceremony) and ikebana (flower arrangement). During this time, he met two members of theIshin Shishi: Todoroki Buhàand Miyabe Teizà Â. After this meeting, he took a serious interest in the concept of kinnà  (å¤çÂÂ), or imperial loyalty.
In 1851, he joined the Kumamoto lord Hosokawa Narimori and went to Edo for his lord's sankin-kà Âtai rotation. It was during his service to the lord in Edo that Commodore Perry arrived in 1853. As the shogunate subsequently entered into a series of increasingly unfair unequal treaties, Gensai left Edo in anger and returned to Kumamoto, where he entered the Gendà Âkan academy of the kinnà  scholar Hayashi à Âen. After a thorough schooling in à Âen's kinnà  philosophy, Gensai returned to Edo.
Gensai was present at the Kumamoto residence in Edo during the Ansei Purge. In the aftermath of Ii Naosuke's assassination, when a group of the escaping assassins suddenly entered the residence, it was Gensai who calmed down the subsequent uproar, calling for a doctor and having a private tea ceremony for the men. It was during this ceremony that he told the men of his admiration for them.
In 1861, Gensai married Misawa Teiko, the daughter of another Kumamoto retainer. A martial artist herself, she was highly skilled in the use of the naginata. The couple had a son, Gentarà Â, who survived even after Gensai's execution, thanks to Teiko's efforts.
In 1862, he joined Kumamoto forces who were posted to security duty in Kyoto. After the political event of Higo-han, he left there and went to Chà Âshà «-han, where he became a personal body guard of Sanjà  Sanetomi. It was at this point, he quit his job as bà Âzu, and soon after, left Kumamoto service altogether. In 1864, he lost his mentor Miyabe Teizà  to a Shinsengumi raid at Ikedaya.
Soon after, Gensai carried out his most famous and only confirmed assassination: that of Sakuma Shà Âzan. Together with at least three other assassins, they ambushed and attacked Shà Âzan in broad daylight on 12 August 1864, and ended with Gensai killing him in one stroke. Shortly after that incident, at the Tenryuji temple in Saga Tenryu-ji, Kyoto, Gensai told his confederates, "It was the first time I actually felt I'd killed someone, the hair on my head stood on end because he is the greatest man of the time." While other assassinations have been attributed to him, only his murder of Shà Âzan can be proven.
After this, he withdrew to Chà Âshà « and took part in the military actions of Takasugi Shinsaku's Kiheitai against the shogunate's Chà Âshà « Expeditions. During the second Chà Âshà « campaign by the Tokugawa regime, he participated for Chà Âshà « and eventually won the battle. However, during action in Kokura, he decisively surrendered to Kumamoto forces, and was imprisoned until just after the Meiji Restoration.
In the aftermath of the Meiji Restoration in 1868, Gensai was released from prison. He changed his name to , and served as a military official and sword teacher for the Kumamoto domain. Because of his harboring of some Kiheitai stragglers under his old comrade Oraku Gentarà Â, he was arrested in November 1870 and later transferred to a prison in Tokyo in 1871. He was later executed by decapitation in , Tokyo on 13 January 1872. He was buried at Ikegami Honmon-ji, Tokyo.
The fictional swordsman and wanderer, Himura Kenshin also known as "Battosai", from the manga series Rurouni Kenshin was inspired by Gensai. The character is a repentant former assassin who has sworn never to kill 10 years after the Meiji Revolution.
The character Kawakami Bansai from the manga Gin Tama takes his name from Gensai.