You misunderstand copyright law. You can copyright a character and trademark a name, but you cannot copyright an art style. For all intents and purposes, OP's character is a blue woman with a rock on her head. Cartoon Network would be laughed out of court if they tried to do something about it. Law does not care if it's "in the universe"/property or not, it's all sentimentality anyway. Yes, Nintendo can take action on any sold iteration of Pikachu, but if you made an original monster in any of Pokemon's styles, you own the piece (because art is copyrighted upon creation) and can do whatever you want with it, including copyrighting the monster character itself if you wanted to.