For all K-2’s social dysfunction (or perhaps his disinterest in organic socialization-who could fathom the mind of a droid?), he knew Cassian better than anyone. He’d seen Cassian commit acts even Draven wasn’t aware of.
On Jenoport, he’d found Cassian staring at his blaster with tears on his face. K-2 had volunteered for a memory wipe in case Cassian’s “continued dignity and service demanded it.”

Rogue One novelization
DONT CRY ABOUT CASSIAN AND K-2′s FRIENDSHIP. DONT DO IT.  (via motleystitches)