The Deceiver

This is why you should always listen to the Smart One.

The person who tries to start a good relationship with someone else, like a lover or friend, but turns out to be someone bad at the end of the story. At least one person is able to see through the Deceiver's facade from the very beginning and makes multiple attempts to warn the deceived, but their warning is always ignored until the last minute when the Deceiver shows their true colors. It only gets more insulting to the Smart One if the deceived argues with them on if the Deceiver is good or not and decides to shut them out for good. If the deceived is especially stupid, they'll continue to fall for the Deceiver's trick even when red flags and alarms start to go off left and right as the Deceiver begins to exhibit more and more behavior that hints that they're the bad guy. Eventually, the Deceiver puts an end to their sham when The Truth Comes Out and they try to attack their prey — only then will the Deceived believe the Smart One instead of avoiding the whole mess they got themselves into if they had listened from the beginning.