Gruk's human mother Celiana was kidnapped from her small rural village by a barbarian orc tribe.
Abused physically and mentally, a forced union caused Celiana to give birth to Gruk. Celiana never told him which orc was his father and certainly none of the orcs in the camp treated him as their son.
From the age he was able to hold a weapon (3) he was "trained" by the orc barbarians, which really equated to being bullied. Gruk was not a full blooded orc (there were many other half orcs in the band, they too were bullied, but most were older than Gruk) and he (and his human mother) often were the brunt of their attacks.
One day during a particularly cruel and strange ritualistic public beating (it was a ritual to Asmodeus, though Gruk had no knowledge of such matters. In fact this was the first time Gruk had seen such symbols. He heard whispers through the camp of strange visitors in the days prior, maybe they were responsible for this new ritual and symbol?) a copper dragon happened to be passing by overhead and stopped to see what was going on.
Once the dragon saw the cruelty of the display (and the asmodeus symbol) it flew into a rage and attacked the orcs of the tribe, freeing Gruk, his mother, and the other prisoners.
From that day Gruk worshipped the dragon (Razamaet) who taught him how to harness Draconic Instinct, as well as taught him the ways of Cayden Cailean.
Once he completed his training Gruk set off into the world to seek glory, taking Bounty Hunting jobs to pay the bills. On one of these bounties he met some folks from the pathfinder society and they convinced Gruk to join. He was happy to help those in need, when his temper wasn't getting him into trouble.
Explore, Report, Cooperate.. Gruk can do that, and maybe find some glory along the way with the Horizon Hunters.