Someone who makes secret plans to do something bad or illegal.
The word 'schemer' makes people think of someone sneaky and not to be trusted.
Examples:
The schemer tried to steal the treasure.
I don't want to play with that schemer.
The schemer was caught by the police.
The cat is a schemer, always trying to steal food.
The schemer had a big plan to take over the world.
The schemer's plans always fail in the end.