Raider

noun

1. One who engages in a raid; a plunderer.
2. A person who takes or attempts to take control of a firm against the will of current management by purchasing a controlling interest of stock and acquiring proxies.
3. A special forces operative; a commando.
4. A warship which is light, maneuverable, and fast-moving.
5. A person who uncovers evidence of improper behavior within governmental or private organizations.