No. Anyone who's a major player you'll want to introduce early on, even if it's just in a minor role. You don't want to spring on a character no one has ever met at the very end. It'll make people angry. Plus, with a murder mystery, you want readers guessing about which character did it. It wouldn't be fair if it's a character they've not met before. Plus, they'll be pretty pissed. No. That's not okay and that's not how mystery books are written.