A Raider

1. One who engages in a raid; a plunderer.
2. (business) 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. (military) A special forces operative; a commando.
4. (military, naval) A warship which is light, maneuverable, and fast-moving.
5. (informal) A person who uncovers evidence of improper behavior within governmental or private organizations.

(plunderer): buccaneer, corsair, freebooter, pirate

(person who uncovers evidence of improper behavior): muckraker, activist