A minor criminal who got lucky. Very lucky. Born on the Imperial Shrine-world of Kringle, and then known as Rosenkrantz, he was one of thousands of POWs taken by the Tau when Kringle was overrun.
Shipped off to an experimental colony where the Tau were trying to encourage all their various POWs to embrace the Greater Good, Rosenkrantz soon became involved in various shenanigans, was tortured almost to death by his own people for something he didn't do, and was so enraged by this injustice he promptly went to work full-time for the Tau.
This didn't go according to plan, either, and Rosenkrantz, now Jak Frost, found himself wandering around the Koronus Expanse at the other end of the galaxy, where he eventually lucked out and was hired on to the starship Rose Tattoo, when his abilities soon saw him earn a position of considerable responsibility.
Not bad going for a traitor and heretic, especially one with such bizarre fashion sense, Imperial tastes leaning more towards 'Over-Elaborate with Extra Skulls'.