The Deceiver

This is why you should always listen to the Brains.

The one 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 Brains if the deceived argues with them on if the Deceiver is good or not and decides to shut them out. 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 to them being 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 Brains instead of listening to them from the beginning to avoid the mess they got themselves into.

The reason why the Deceiver wants to lure their victims into a relationship trap varies from character to character. Sometimes it's to feast on the dupe (like with some real-life animals), other times it's to rob them of their good when they're the most vulnerable to attacks. Whatever their motive may be, they all have one thing in common: they can at least trick one person into thinking they're a good guy and take advantage of that tricked person so the Deceiver can get what it is they want.