Probably a Blackshield (A Deathwatch marine who has cut ties with his chapter). There are chapters which use black for the right pauldron, but you should be able to see at least a bit of his chapter's emblem from this angle with how big they are.
Say isn't Mashiro Don Quijote? The hat is the same with the little missing part, spear, shining knight armour and on the horse back. Do we have a tag for it? The only ones about him are the fate and project moon ones.
