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, Guardsmen (and 40k fans) believe strongly in Pius and the virtues his tale extols.
The problem
The problem is that that's all Pius was ever mentioned as: a legend. In spite of this, the fandom, drunk on Humanity Fuck Yeah, has grown to conflate the story of Pius with later, more concretely detailed portrayals of Horus' duel with the Emperor, mistakenly believing Pius was originally objectively present at the battle in its final form only to be 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, written shortly after the duel in the bunker. -
The second telling of the duel in the bunker. Since Terminators are the ones who intervened, Pius' short-lived ambiguous existence was confirmed as nonexistence. -
The source of the claim that Pius' sacrifice motivated the Emperor to take action against Horus. Leaving aside the unlikelihood that a regular human would have been taken into the heart of Chaotic resistance, one surviving an environment lethal even to Terminators would be impossible.
Black Library
In Dan Abnett's Horus Heresy novel Know No Fear, Abnett decided to throw the fandom a bone and made Ollanius Pius a demonstrably real person, albeit in a way at odds with the image the fandom's built up of him.
The man behind the legend is one Ollanius Persson "the Pious" (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 roughly 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). This, along with the psychic gestalt ubermensch that is the Emperor, is part and parcel of how Oll and the Emperor are thematic antitheses to each other: while the Emperor has always been a grand leader with pivotal roles all throughout history but ultimately grew alienated from mortals, Oll has always been a follower, just one among many, and has retained his ability to identify and empathize with the people around him.
In the days of ancient Terra he and the Emperor were two of the Argonauts who adventured with Jason to get the Golden Fleece, and later he fought for the French during World War I. His bayonet fighting skills are demonstrated at Verdun in Angel Exterminatus, when the Iron Warriors Trident Warsmith Kroeger relives (sort of, the memories were the memories of other people) the near-death of "Carl", a German soldier, at the hands of one "Olivier Persson". Carl thought Oll was very rude because he interrupted Carl's dinner. Yes, poor Carl's dinner. Oll 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 pious asshole to stab him in the gut. Carl was only saved from a certain death when the crush of the battle forces Oll out of the trench.
He is confirmed as being very religious (the last known "Catheric" in the galaxy, in fact) but hates Chaos and looks to the Emperor as a peer rather than a god. 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 Oll by Grammaticus indicates that he'll be the person who sacrifices his life to save the Emperor from Horus, finally, irrefutably canonizing the legend of Ollanius Pius after many years of dubiousness and impossibility. 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.