You can, but you need to be careful.
In a prior post on the character of Sherlock Holmes I made two important points about using pre-existing characters:
- Where there are portions of a character in the public domain and portions that aren't, it's permitted to fork the characters to use only the public domain elements.
- Forking characters in this way doesn't infringe on the rights of a copyright holder who might own non-public-domain elements to those characters.
But what may not be clear is how you can take a public-domain character and make them your own creation such that no one else can use your version of them, and in contrast when you're making too close use of someone else's version of the characters. A recent judgment from Canada gives us 3 good rules to help you make that decision.