Background
SH-1P ("Skippy" to friends) was a protocol droid belonging to an eccentric researcher, who employed him primarily as a chauffuer and translator. After the researcher's tragic death following a badly-timed holiday to Alderaan, SH-1P was auctioned off as part of her estate, and found himself bought by a high-ranking Alliance intelligence officer who immediately set him free. He chose to continue in the ranks of the Rebellion and now regularly flies "crews that can't fly themselves" as he so kindly puts it.
As a result of more than a little mind-addling from his previous owner, SH-1P has a split personality - one gritty and fearless and the other timid and neurotic. The gritty personality often makes vague references to tragic or implausible events from his past - which, as far as anyone can tell, never actually happened.
Motivation
VENGENCE: While he claims to work for the joy of it, and out of a sense of gratefulness to the man who frees him, deep inside SH-1P craves the downfall of those who destroyed Alderaan and his former employer. He rarely engages directly with the Empire, however, as he believes the best way to defeat them is to contribute his talents to the Rebellion effort, which he does with enthusiasm.