Manji certainly doesn't look like someone that would be picked for a Rebel commando operation, but that's part of the point. It's easy to stay fresh-face and unscarred in the era of bacta, especially when you haven't seen 30 standard years yet. He's seen a lot in that time, though.
A native of Coruscant, raised on Imperial propaganda and the victory in the Clone Wars, he gladly signed up for Imperial service to 'preserve Galactic peace' a year before regulations strictly allowed. He was not trained as a stalwart stormtrooper, but instead to be quiet, weapons of imbalanced destruction and terror. But it was for the good.
That held until the Empire's attack on Dac - a previously peaceful and long-time member of the Republic were to be placed into chains. He doesn't talk much of what happened there, but is always a bit uneasy among the Mon Cal members of the Alliance.
It was only after that failed campaign, when he had downtime to process and think about everything that had happened, that led to his change of heart. On his next assignment in the Mid Rim, he vanished, appearing with the Rebel cell he'd been tasked to capture. He served with the cell for a few years before joining the larger Alliance in the run-up post-Yavin
He's relaxed a lot since then, and even can laugh a bit, in the two years since joining the Alliance to Restore the Republic. Maybe when the Galaxy is again united he can find something more constructive to do, but sadly the skills the Empire trained him with are just as valuable to the Rebels, at least for the foreseeable future.
