From PSU: “Genichiro Ashina is the third boss in Sekiro: Shadows Die Twice. He is found at the top of the tower in the Ashina Castle area. You can reach him by heading along the rooftops and around the left side. At the back of the tower you can grapple up to enter it. At the very top is Genichiro.
He can be a tough fight due to the variety of attacks he has. He also is the first boss to have three healthbars. He also has two separate phases; the first phase take up two healthbars and the second takes up the final one.”
Source: N4G PC Sekiro: Shadows Die Twice Genichiro Ashina Boss Guide – Ashina Castle