From the Profile itself in the Appearance in FGO section:
The owner of a non-standard talent with the sword that could imitate any fencing just by seeing it, Okada Izō.That even fearsome natural talent released Izō into the world of the Bakumatsu[1] as a matchless hitokiri.
Famed as a master of Tenchū[2], Izō was a prominent Assassin in the history of Japan that participated in numerous assassinations, but couldn’t attain greatness as a swordsman because of that.
The Bakumatsu was the era in which the act of 「cutting down people with a katana」 was performed the most, and the sword technique of the 「hitokiri」 who represents that era, Izō, is comparable to the Saber class despite being an Assassin.
From Wikipedia.
https://en.wikipedia.org/wiki/Four_H..._the_BakumatsuOkada Izō (岡田 以蔵, February 14, 1838 – June 3, 1865) was a Japanese samurai of the late Edo period, feared as one of the four most notable assassins of the Bakumatsu period. He was born in Tosa to the gōshi Okada Gihei, who had been a peasant but had bought the gōshi rank. Izō and Tanaka Shinbei were active in Kyoto as assassins under the leadership of Takechi Hanpeita.