Yes you are correct that +Res -Def isn't the best but it isn't the worst. Corrin(F) with those stats can still survive a OHKO from all Falchion users so it isn't too bad. If you want your Corrin(F) to be more resistant towards mages and other dragons then +Res -Def would be okay. If you don't, then I would recommend promoting the free Corrin(F) you got from Android.
Which Corrin(F) should I use and promote to a 5*?
I have 2 female Corrins and I really want to use her on my team. One has +Res -Def and the other is the one that was given out to all Android users, so all avg. I'd like to promote one to five star but I'm not sure which one, +Res -Def isn't really the best.