Donitz has allowed creative freedoms with the characters. While Mitta's template isn't my creation, her personality and the history I've invented for her are. I guess you could say it's similar to adoptables.
Creative freedoms aren't the same as owning a character. Her personality may be your own, but it's still Donitz's character. It's comparable to my portrayal of Majora, a Zelda villain. I gave him a different personality and background, but he's still, at the end of the day, Nintendo's character. See what I'm getting at?
The issue here is copyrighting. Donitz hasn't claimed rights to the character, while Nintendo spells out in detail the copyright of Majora and all of their videogame characters. That, and I've stated where Mitta originated from each time I put out new art involving her. That said, I do understand what you're getting at.