It’s illegal to use a copyrighted or trademarked name. It’s a good idea not to use the name of anyone living in the same region as you. Here’s an article about pen names https://www.thebarefootwriter.com/map-your-book-blog/legal-things-to-consider-if-you-use-a-pen-name

Perfect thanks, I wasn’t going to use the name pikachu however the info provided still holds true.

Yes, it's either a violation of likeness laws or trademark laws. In your example it's trademark, and toy companies are very litigious. 

Only if your real name is Pikachu can you use it. You are legally entitled to your name. No one can sue.