This will be a little lengthy, but hopefully informative about commissars.
I think you got the character pretty correct, but I wouldn't describe him as large. I picture him more like tall and lean, dressed similar to the 40K commissar models. Let's see if I can find a picture...
or 
, just without the mask, and with different weapons. You've got to love that uniform.
The Imperial commissars are pretty much the same as the Soviet Red Army comissars: Accompanying officers to make sure they act in a correct way politically (and religiously, this is the Imperium), and maintain discipline. Ah, whatever. I'll just ccut and paste the part on Comissars from the fluff bible. In 40K, there's a special rule for commissars: If the unit they are with falls back, they execute the officer of the unit, and then the unit must take a Ld test and either the commissar assumes command, or they frag him and run.
Quote:
Commissars
Imperial Guard regiments are recruited from all over the human galaxy. Warriors from one planet speak different, sometimes unintelligible, dialects or practice strange customs which are baffling to soldiers from other worlds. These vast differences make it hard for some regimental commanders to operate closely with troops from different worlds. If colonels, captains and lieutenants are to function as part of a cohesive army they must be united in their common purpose no matter how culturally diverse their backgrounds might be.
Commissars provide the link between regimental commanders and strategic officers. Commissars have to be tough. Some regiments are composed of savage former gang fighters, or vicious barbarians, who are naturally antagonistic to authority. The colonel of such a regiment is likely to be as wild and anarchic as his men, if not more so! The loyalty of such troops must be earned and the Commissar must be prepared to demonstrate his courage in battle.
Commissars are recruited into the Departmento Munitorium from the Schola Progenium, the rigorous orphanages for families of Adepts run by the missionaries of the Adeptus Ministorum. Many serve as Storm Troopers before becoming Commissars, so they are hardly strangers to warfare. Every regiment has at least one Commissar and many large regiments have several who remain with the regiment while it is deployed in a war zone.
The chief task of the Commissar is to preserve the fighting spirit and loyalty of the regiment. If discipline is lax the Commissar will step in to reinforce it. If the regiment's officers are incompetent or lack courage the Commissar must retrain and fortify them or, failing that, allow the regiment's squads to be dispersed to other commanders. If troops are rowdy and troublesome the Commissar must keep order. A Commissar knows that the best troops are the hardest to keep in line. He embodies strength, bravery and loyalty, and serves as the ultimate example of human courage.
Commissars are also zealots fiercely loyal to the Imperial Cult. In the Schola Progenium they are schooled to love the Emperor and desire nothing more than to serve to the best of their abilities. They tolerate no disloyalty and remain vigilant for spies, mutants and agents of chaos that may have infiltrated the men under their charge. They are versed in the Imperial scriptures and will frequently give readings before battle is joined. Most of all they despise the Emperor's enemies and desire nothing more than the chance to crush the foe beneath the righteous heel of Imperial might.
Imperial Guard regiments are recruited from all over the human galaxy. Warriors from one planet speak different, sometimes unintelligible, dialects or practice strange customs which are baffling to soldiers from other worlds. These vast differences make it hard for some regimental commanders to operate closely with troops from different worlds. If colonels, captains and lieutenants are to function as part of a cohesive army they must be united in their common purpose no matter how culturally diverse their backgrounds might be.
Commissars provide the link between regimental commanders and strategic officers. Commissars have to be tough. Some regiments are composed of savage former gang fighters, or vicious barbarians, who are naturally antagonistic to authority. The colonel of such a regiment is likely to be as wild and anarchic as his men, if not more so! The loyalty of such troops must be earned and the Commissar must be prepared to demonstrate his courage in battle.
Commissars are recruited into the Departmento Munitorium from the Schola Progenium, the rigorous orphanages for families of Adepts run by the missionaries of the Adeptus Ministorum. Many serve as Storm Troopers before becoming Commissars, so they are hardly strangers to warfare. Every regiment has at least one Commissar and many large regiments have several who remain with the regiment while it is deployed in a war zone.
The chief task of the Commissar is to preserve the fighting spirit and loyalty of the regiment. If discipline is lax the Commissar will step in to reinforce it. If the regiment's officers are incompetent or lack courage the Commissar must retrain and fortify them or, failing that, allow the regiment's squads to be dispersed to other commanders. If troops are rowdy and troublesome the Commissar must keep order. A Commissar knows that the best troops are the hardest to keep in line. He embodies strength, bravery and loyalty, and serves as the ultimate example of human courage.
Commissars are also zealots fiercely loyal to the Imperial Cult. In the Schola Progenium they are schooled to love the Emperor and desire nothing more than to serve to the best of their abilities. They tolerate no disloyalty and remain vigilant for spies, mutants and agents of chaos that may have infiltrated the men under their charge. They are versed in the Imperial scriptures and will frequently give readings before battle is joined. Most of all they despise the Emperor's enemies and desire nothing more than the chance to crush the foe beneath the righteous heel of Imperial might.