Faithful readers will have gotten a Death Star memory core's worth of new information about Rogue One: A Star Wars story in the latest issue of Empire (in all good and evil newsagents and also available here{
The Astromech droid (who sadly didn't make it to the photoshoot for our cover, leaving the way clear for K-2SO to steal the spotlight), is in service to those nasty Imperial types, and is used to maintain ships and other equipment. But unlike R2-D2, this unit isn't granted independence and is apparently subject to regular memory wipes. It's similar to what we do to those who come into the office for work experience. Wait... Forget we mentioned that. (Our workies already have).
As for how this little unit fits into the story... We'll have to wait and see on that point. Rogue One: A Star Wars Story is out in the UK on December 16.
Read our feature on Everything You Need To Know About Rogue One