5. IG-88
What you see in the films: IG-88 is one of the bounty hunters in the infamous lineup scene in The Empire Strikes Back. Darth Vader has summoned a "murderers row" of notorious bounty hunters to hunt down and capture the Millennium Falcon. He's an ominous looking droid, draped in weaponry.
The expanded backstory: IG-88 is an assassin droid. As good a bounty hunter as he was, it was all a cover for the original IG-88 (called IG-88A), and his master planthe Droid Revolution. Upon his activation, IG-88 killed his creators and transferred his consciousness to three other IG-series droids, creating IG-88B, IG-88C, and IG-88D. The four plotted a Droid Revolution which was exactly what it sounds like- kill everything that isn't a droid. It was actually IG-88B who appeared in the Vader scene with the other bounty hunters. The highly competitive Boba Fett ends up killing IG-88B on Bespin, relegating his remains to scrap metal. Fett also takes out IG-88s C & D, leaving IG-88A as the sole survivor. He effectively uploads his consciousness into the Death Star, and plans to beam out the activation command for all droids to turn on their masters, but the rebels destroy the Death Star before he can carry out his task.