Oh hey it's an Actual Real Dude being used as this dude's ancestor/previous life/whatever. A Samurai and talented military reformer whose innovative military organization lead to major victories against the Tokugawa Bakufu which ultimately resulted in the Meiji Restoration. Sadly he would not personally live to see this, having died of Consumption like a Victorian Heroine the year before at the age of either 27 or 28, the Wikipedia lists both ages in different locations.
hang on, ancestor? or immortal? it'd be kinda fitting, an immortal witch swearing off dating then ending up breaking that oath and getting hooked with an immortal guy she first met so long ago