Ollanius Pius
Ollanius Pius is a folkloric saint of the Imperial Guard. As the legend goes, during the Siege of Terra Ollanius Pius, a soldier in the old Imperial Army, protected the God-Emperor of Mankind by throwing himself between his master and the single greatest champion of the Ruinous Powers, taking a mortal blow meant for the Emperor. Even to this day, the Imperial Guard believe strongly in this story and the virtues Pius embodies.
The problem
The problem is that that's all Pius was ever written as: a legend. In spite of this, the 40k fandom, drunk on Humanity Fuck Yeah, has grown to conflate the legend of Pius with later, more concrete portrayals of Horus' duel with the Emperor, mistakenly believing Pius was retconned away. As you can see below, this is not the case.
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The first version of Horus and the Emperor's duel. Notice the location, making Pius' sacrifice possible, if implausible. -
The legend that spawned the myth. -
The first version of the Siege of Terra that ended on a ship in orbit, and the source of the claim that Pius' sacrifice motivated the Emperor to take action against Horus. It is highly unlikely a regular human could have survived the bowels of the Vengeful Spirit, given how deadly it was to Terminators. -
Fantasy Flight Games, blissfully unaware of Pius' confirmed existence...
Black Library
In Dan Abnett's Horus Heresy novel "Know No Fear", Abnett decided to toss the fandom a bone and make Ollanius Pius a demonstrably real person, albeit in a way a bit at odds with the image the fandom's built up of him. He is identified as one "Ollanius Perrson" (or "Oll" to his friends) and is part of a small group of immortal humans spread throughout the galaxy called Perpetuals. John Grammaticus (who is alive and well) claimed that out of the entire Ultramarine empire (which at the time consisted of a staggering 500 planets) there are only three Perpetuals (the total amount in the Imperium is unknown, but almost certainly not much higher). Pius is confirmed to be at least 30,000 years old and is the same "breed" as Grammaticus and possibly the Emperor. Besides his extreme age and the whole immortality thing he seems to just be a normal human with normal human strength (coupled with the fact that, by all indications, he hasn't died once so far). Which, when compared to the psychic gestalt ubermensch that is the Emperor, is kind of important: Ollanius is more representative of every strength (and weakness) of mankind than the Emperor EVER was.
In the days of ancient Terra he was one of the Argonauts who adventured with Jason on the Argo to get the Golden Fleece and later he learned how to fight with a bayonet whilst fighting for the French in the trenches at Verdun during World War I. He actually demonstrates his bayonet-trench fighting skills at Verdun in the novel "Angel Exterminatus". In it, the Iron Warriors Trident Warsmith Kroeger has a flashback (sort of, the memories were the memories of other people) and relives the near-death of "Carl", a German soldier at the hands of one "Olivier Perrson." Carl thought Ol was very rude because he interrupted Carl's dinner. Yes, poor Carl's dinner. Ol happened upon Carl eating... Carl's own commander. Oh yeah, Carl was a closet cannibal. Still, Carl thought it was a bit of an overreaction by the overly Pius asshole to stab him in the gut. Carl was only saved from a certain death when the crush of the battle forces Ol out of the trench.
He is confirmed as being very religious but hates Chaos and looks to the Emperor as a peer rather than a god/divine. He was a veteran in the Imperial Army before retiring to become a farmer on Calth. In the events of Know No Fear Pius joins in the fight against Chaos.
A vision given to Ollanius by Grammaticus indicates that he'll be restored to his original status as the person who sacrifices his life to save the Emperor from Horus, fully realizing the legend of Ollanius Pius. During Mark of Calth he realizes that this is going to be the end for him. Which makes sense given that he was killed by a super-Chaos Horus. You don't get up from that, immortality won't help you if your soul is snuffed out.