Ollanius Pius, the manliest ************ you ever did see.
During the Horus Heresy, this guy showcased exactly what it means when people say that Imperial Guardsmen have balls of steel. Why the Emperor was having a dialogue with Horus, even after Sanguinius had been slain, Emps still felt that there was good in his favorite son and that he could yet change his ways, the eternally loving father that he was. As Horus advanced on Big-E, ready to strike, the Emperor did nothing, still trying to talk Horus down.
A single guardsman, no genetic modifications or superweapons (His lasgun was absent, too, so he was without even his trusty flashlight), stood in between the two Gods-On-Earth, refusing to move. Horus told him to step aside, yet Pius stood his ground. Naturally, Horus simply killed him to remove the tiny pestering man. The Emperor, now seeing that the lives of the citizens of the Imperium, especially one that served in the noble Imperial Guard, meant nothing to him and that Horus was beyond redemption.
The Emperor then proceeded to dish out the single greatest psychic ass-whoopin' the 'verse had ever seen.
Without Pius and his balls of fuckmothering Impervium, there would have been a significant difference in the discourse of the battle between Horus and the Emperor. This dude is the epitome of the Imperial Guardsman. Seriously. He steps himself in front of the single most powerful Chaos entity to enter the materium and refuses to step aside, simply because he did not want any harm to befall the Emperor. Seriously, even Teddy Roosevelt would want to get this guy's autograph.
Though, Games Workshop being retarded as they are, naturally retconned their awesome canon and replaced Pius with an unnamed Imperial Fist Space Marine, and later an unnamed Adeptus Custodes.
Regardless, he still remains the manliest ************ you ever did see.
A very informative, albeit short article on Pius: http://wh40k.lexicanum.com/wiki/Ollanius_Pius
A much more humorously bent article, though one that captures the patron saint of the Imperial Guard in a much better way: http://1d4chan.org/wiki/Ollanius_Pius