There's nothing creepier for a character than knowing there is a stalker out their watching their every move. Maybe they send gifts, or notes, maybe they try to hurt people close to the character. Either way, the character ends up feeling vulnerable, and often this includes the introduction of a cop/agent etc.
One of my most recent books, Guarding Garrett (Hockey Romance) features a stalker with a twist. One of my favorite books to write was the Ex Factor – a stalker/bodyguard romance.