How Can I Legally Protect My Fictional Characters?
You can protect your fictional character through copyright and trademark law.
Copyright law protects original works of authorship that are fixed in a tangible medium of expression. That means your work has to be original (you created it yourself), fixed in a medium (e.g., saved in Microsoft Word) and offers a modicum of creativity (a scintilla of creativity --- for example, a list of names in a phonebook is not creative enough because it just lists names in alphabetical order). If your novel is original, fixed, and has minimal creativity, your novel will be copyrightable. The only thing you need to do is register your copyright to be fully protected.
But what about your characters? The same logic applies. If your characters are distinctive enough, they can be copyrighted. Now if your characters are more like "stock" characters in a particular genre, they will likely not qualify for protection. So try to make your characters as unique as you can.
Trademarks are a word, phrase, or logo that identifies the source of goods or services. Essentially, trademarks help you as a consumer distinguish different goods and services from one another. For example, the Nike swoosh lets you immediately know you're buying a Nike sneaker and not a generic brand. The theory is that by awarding companies trademarks it will incentivize them to create high quality products and services that you will come to identify them with. It produces goodwill for the company.
So how do trademarks work with books, specifically with characters? You can seek to register a trademark in your character if it is unique enough. However, please note that the trademark protection will be limited to how your character is used in commerce within certain classes of goods and services (for example, if you want to put your character on a t-shirt or on a mug). Trademarks won't protect your character in and of itself.
However, the big takeaway is that to get a trademark registration, the public needs to associate your character with its source (a.k.a. you, the author). If there are multiple characters (or authors) with the same name or similar character designs this may be difficult. This is partly why trademarks are usually only registered for characters that have gained a certain level of notoriety or fame from the general public (e.g., Harry Potter, Spiderman, Mickey Mouse). In trademark law, we call the notoriety "secondary meaning."