Synonyms of 'conspirator'
A plotter is a person who secretly plans with others to do something that is illegal or wrong, usually against a person or government.
the chief plotter behind the unsuccessful coup attempt
If you refer to someone as a schemer, you mean that they make secret plans in order to get some benefit for themselves.
She is a schemer, my wee sister.
Thesaurus for conspirator from the Collins English Thesaurus
A conspirator is a person who joins a conspiracy .