R is for R5-D4
So what if R5-D4 didn't blow his motivator - would Luke still be stuck on Tatooine? Would R2 have been melted down, ruining his 18 years of planning? Would the Emperor be unstoppable? We shall never know.
Some other characters, droids, creatures and species that start with R:
RA-7 Death Star Droid
Roose, Ark "Bumpy"