A ninja assassin. Need we say more?
Don't worry—we will. This guy's vat grown and works for the Tessier-Ashpool family, particularly 3Jane since she is currently in charge of the family affairs. He works as a bodyguard and even does wet work when needed (i.e. he kills people). His Zen mastery allows him to fire his bow and arrow even when blinded. Yep, he just got even cooler.
We don't see a lot of Hideo, so it's difficult to know how much of what he does stems from loyalty to the Tessier-Ashpool family, a code of honor, the pay, or perhaps even love. But in any case, he's not to be trifled with.