K2 SO was an enforcer droid originally created for use in the Galactic Empire's armed forces.

At some point, however, the droid encountered Captain Cassian And or -- a member of the Alliance to Restore the Republic, a resistance movement that sought to overthrow the Empire and restore democracy. And or was able to reprogram K-2 SO to serve the Alliance

Along with a band of roguish Alliance operatives, K-2 SO went on a daring mission to steal the plans for the Empire's moon-sized battle station, the Death Star

Rogue One: A Star Wars Story