In most cases, using a character's name from a fictional universe is not a trademark infringement (unless the character is being reused, that would be a copyright infringement). Why does this happen?
First, a trademark has to be registered with an authority, such as the US Patent and Trademark Office, to be recognized as such. One does not automatically gain trademark protection, it has to be obtained by applying for one. Which is not automatically approved, by the way.
Second, even if a name is trademarked, a trademark does not work the same was as copyright. Trademarks are mostly about protecting your brand, that is, ensuring the public knows a particular name or image is associated with you, or your company, and not a competitor or imitator. It's not really concerned with preventing other people from using the same name, or image, just using it in a way that would "confuse the public" in regards to authenticity or origin of it.