Shimotsuki Ryuma

Ryuuma is a Zombie General who looks like samurai. He appears to have some kinda of tie with Brook.

History
According to Hogback, Ryuuma hails from the Wano Country in the New World. At some point in time, he apparently died and was reanimated by Hogback to be a guardian of sorts.

While apparently doing his rounds on the night the Straw Hats came to Thriller Bark, Ryuuma decided to chase after Usopp, Nami, and Chopper after they were chased out of Hogback's room by a Jack in a box. He caught up to them while they were peeking on Hogback making his latest creation in his lab. He then knocked them inside with what appeared to be slash.

He knocked Usopp, Nami, and Chopper away with a single move and walked 10-feet away to converse with Cindry before they even realized they were attacked.

Pre One Piece Ryuuma
Ryuuma's character design, past, and even name appear to be based on a character from one of Oda's earlier works, Ryuuma the King.