Zangetsu is a character I remember in Curse of the Moon but his role here is much different, though similar. He appears to work for other characters, but he has his own motives.
He’s one of the first bosses, and since you have few powers and low health at this point, he’s quite challenging to beat.
But he later allies with Miriam.
You even fight a boss together, sort of.
You fight him a final time. He says it’s a test, for which he gives you his katana sword as a reward.
Not the most powerful weapon in the game, but you use it to destroy a fake moon in order to get to the true ending of the game.
At the end Zangetsu appears to have been destroyed by Dominique’s magic, but I hear he’ll most likely come back as there’s hints of it in the ending.