Rex and ROX-N both worked for the Star Tours agency, albeit in different posts. Rex was a pilot droid. ROX-N was a protocol droid. They were together. And they were happy.
And nobody cared, provided they didn't mess up their jobs.
Then came Rex's first flight.
It went… memorably.
As Rex relayed the events to ROX-N, he couldn't help but embellish some details - only slightly. It wasn't as though he didn't help the New Republic destroy some Imperial Star Cruisers and a Death Star - he had.
ROX-N was so proud of him.
Only Rex knew it had been an accident!