Originally a KX-security droid for the Empire, K-2SO played a key part in the destruction of the first Death Star. Before being reprogrammed, K-2SO was one of many security droids deployed by the Empire on Ghorman during its massacre of citizens in Palmo. As the KX droid was about to attack Cassian Andor, Ghorman Front member Samm rammed it with a transport, splitting the droid into pieces.

Cassian took the droid parts to the rebel base on Yavin 4, where a tech reprogrammed and rewired the KX-security droid. After being awakened, K-2SO lost his Imperial programming and violent impulses, but was unable to filter his blunt thoughts and observations.

Afterward, Cassian and K-2SO – Kay for short – frequently teamed up on missions. Kay accompanied Cassian and the rebel soldier Ruescott Melshi on a mission to Coruscant to rescue Luthen Rael, despite protesting the number of regulations they were breaking in the process. Kay successfully took out an entire Imperial tactical team just as they were approaching Luthen’s safehouse, where Luthen’s assistant Kleya had been broadcasting a call for help.

Shortly after, Kay and Cassian headed to the Ring of Kafrene, where a rebel spy gave them intel about a secret Imperial superweapon. He later helped free Jyn Erso from an Imperial prison, and accompanied Jyn and Cassian to Jedha and Eadu as they learned more about Jyn’s father’s involvement in the creation of the Death Star.

Kay again joined Cassian, along with Jyn, Bodhi Rook, Chirrut Îmwe, and Baze Malbus, as part of the Rogue One mission when they disobeyed orders and left Yavin for Scarif for a chance to steal the Death Star plans. After landing, Kay led Cassian and Jyn to the Scarif vault. As Imperial troops approached, K-2SO secured Cassian and Jyn in the vault, giving them the information they needed to find and transmit the plans to the rebel fleet before he was overwhelmed by blaster fire.