Greiss is a rarity.
He was not, like most of the Imperial Guard's greatest, a pinnacle of human physical achievement, nor was he a tactical genius. Greiss was a Commissar attached to the Catachan 2nd Regiment. As Catachan is the planetary equivalent of Dwarf Fortress in terms of day-to-day survival, Greiss fought in the toughest environment imaginable and somehow, survived. Widely-known for his gaunt body and distinctive officer's cap, Greiss had to fight hard to earn the respect of his regiment - which is tough when you consider that most Catachan units treat Commissars like /tg/ treats threads started by retards.
Against all odds, Greiss proved both a capable and effective leader, one much more understanding than the norm brought about by the Commissariat (which was likely for the best considering how often Commissars "go missing" with the Catachan regiments), and following the death of the Catachan 2nd's previous commanding officer, stepped up to the plate and took charge of the regiment, to unanimous approval by the troops. He would go on to lead them through campaign after campaign, and would eventually be succeeded by his subordinate, Colonel "I Just Punched a Mekboy's head off" Straken. Like his successor, Greiss knew his place was at the front line, and would not back down, even given his comparative frailty compared to his men, preferring to lead by example - proving that though he was not the biggest and strongest, he had the testicular fortitude to stand with the greats.
He retired after being critically injured on one lengthy campaign, turning the Catachan 2nd over to his successor, who was Colonel "I wrestled a Bloodthirster of Khorne to the ground and ripped its spine out" Straken. He then gave his Commissar hat to his bodyguard, which became his prized possession.
Greiss and Ogryns
Greiss is perhaps best-known for his open acceptance and encouragement of the use of abhumans, specifically Ogryns, in the Catachan 2nd. Most officers balk at the prospect of working with the smelly grunts, but Greiss immediately saw their value and use when he saw just how vital their talents could be in the Catachan 2nd. So impressed was he that he eventually took on an Ogryn as a personal bodyguard, Nork Deddog, an Ogryn of legendary fame who is still around to this day. The camaraderie of the officer and Nork is well-known; countless were the occasions where members of the Catachan 2nd could recall moments wherein Greiss was screaming orders whilst ammo bounced off Nork's thick skull. Especially famous was one assignment wherein Greiss was critically wounded when a shell tore through his Chimera, and Nork responded by dragging the immobilized Chimera all the way back to the rest of the regiment in order for Greiss to receive medical treatment.
Greiss liked Chimeras. A lot. As he was supposedly brought to Catachan from Armageddon, this comes as no surprise, but Greiss' tactical doctrine wound up working nicely with the general offensive principles of the Catachan 2nd. Chimeras in the 2nd were equipped with rolls of Camo netting in order to keep them hidden, and made excellent mobile firepoints. The fact that they were amphibious only made them more useful. All in all, the addition of numerous Chimeras to the Catachan 2nd gave it considerably more flexibility than it had before.