In my current lists, he leads a squad of 20 cultists, and has already achieved infamy in a few games (for all the wrong reasons).
In my local gaming circle, he has been lovingly named Arnold Porkinson, and I've dreamt up a bit of background for him.
Porkinson is the disgraced son of a planetary governor of an Imperial garrison world somewhere in the Ultramar segmentum. As the second son of a noble in this region of the galaxy, he was tithed to the local Imperial Guard Finishing School for Officers, where he soon excelled, and was earmarked for a career as a morale officer with the Commissariat.
Again he excelled in his training, until one fateful day of training exercises and live-fire battlefield simulations. On that fateful day, he was set up by a jealous cadet, one Giles Coriandus, who took it upon himself to trip Porkinson whilst simultaneously shooting the instructing Commissar in the back with his bolt pistol. In the ensuing confusion, Porkinson and Coriandus' bolt pistols were mysteriously swapped, and the blame thus pointed at the seemingly clumsy Porkinson.
Despite his protestations of innocence, one severely maimed Commissar later, Porkinson was unceremoniously discharged from both the Commissariat and the Imperial Guard Finishing School for Officers AND the Imperial Guard itself. He was subsequently sent back to his father's holdings in disgrace. His father of course was outraged. Officer or not, Commissar or not, the second noble son must serve as a soldier, in some capacity or other. Thus Porkinson was required to enlist in the local Planetary Defence Force.
However, the local PDF is no place for a noble's son, as he was soon to find out. The PDF of course is made up of grunts who just can't cut it in the Imperial Guard, along with thuggish miscreants who have failed to find paid employ elsewhere. He was victimised and bullied by his new found peers until he proved himself worthy of their respect by busting heads and breaking noses in a barracks brawl. He even rose as high as the rank of Sergeant several years later, and it seemed he had managed to find a niche after all and redeem some honour for himself.
That is of course until a chaos cult emerged on planet and the subsequent spread of sedition. Eventually, an offshoot of an invading chaos fleet arrived in the system and soon made planet-fall. The Imperial Guard were called in to help put an end to the invasion, and this was when Porkinson and Coriandus were to cross paths again.
It seemed that Porkinson and his PDF unit had been tempted by the seditious whisperings and pamphlets that had been distributed by the chaos cult, and thus when they found themselves backing up a brave charge by the Imperial Guard, which was being led by the dashing (and smirking) Commissar Coriandus, there was another "friendly fire" accident. All Porkinson's bitterness, resentment and jealousy at events from a few years back once again bubbled to the surface once again, and one well placed las bolt to the back of the head later, Commissar Coriandus smirked no longer, and Porkinson took Coriandus' greatcoat and cavalla sabre as his battle spoils.