The Down & Out (EotE, pg 36)
Grenn wanted to be a pilot so badly he signed up for the Imperial Naval academy when he was old enough.
They booted him out unceremoniously when he carelessly topped an unsecured TIE Fighter wing onto the academy commandant's prized classic pre-Empire landspeeder.
The personal insult to the commandant resulted in his ejection, it had nothing to do with his ability. He was, and still is, a great pilot.

(continued from Background)
His motivation is Spite:
He will run escort pilot runs for almost anyone opposed to the Empire. He's got no love for them, the petty bastards, so he's happy to operate out of spite, which is more than enough to land him jobs from the Rebellion, whether or not he's aware of it.
He is known to the Rebellion, as someone who has enough personal distaste for them that he's only happy to stick it to them when he gets the chance, even in small ways. That kind of vendetta is trustworthy.

Space Superiority
Support

A human male, approximately 175cm tall, with brown hair and brown eyes.
He wears a comfortable flight suit, in fairly drab colours (browns, greys, and blacks), adorned with a few decorative buttons and several repair patches, partly to have his own look while not standing out too much, and partly because he's a bit too cheap to get a new flight jumpsuit.