If you have your work copyrighted you can. But since you dont even have a watermark on your files… I dont think you can do anything.
You could get a lawyer and send a cease and desist letter for starters… but being you yourself dont have permission to copy right the characters… Im not sure.
If it were your own character creation you would definitely need copyright and watermark all your pics.
But to do someone else’s work via your interpretation… you would need there written permission first before you were able to watermark and copyright it as if it were yours.
Its really no different than a band doing a cover song and selling it for a Toyota commercial… uhhhh… not gonna work… original artist is going to take a good chunk of that money. And if you dont ask for their permission…they will get sued.