double agent (n.)

• betrayer
• cockatrice
• conspirator
• counterspy
• double-crosser
• double-dealer
• informer
• intriguer
• machinator
• operative
• plotter
• quisling
• rat
• schemer
• scout
• serpent
• snake
• spotter
• spy
• timeserver
• traitor
• treasonist
• trimmer
• turncoat