The Hero

The Hero is a character mentioned in Doom 3 and discussed in Site 3's Chamber of the Hero.

Background
He was thought to be an ancient Martian who was the only one known to be able to wield the Soul Cube into battle against the Demons of Hell (and the demon who birthed them all).

His empty sarcophagus can be found in Site 3. Where the archaelogist Pierce Rogers had been studying it and the four stelae near it.

Appearance
Based on the stelae/tablets he wore armor not unlike those of UAC Marines in Doom 1/2/3 and Doom 64.

Behind the scenes
The Hero and his appearance are nods back to the Doom Marine from Doom 1 and 2. The artwork on the stelae being based on the box art of Doom 1.

Doom (2016) shows similar artwork depicting Doom Slayer connected to the Slayer Testements, and another stelae depicting Doom Slayer wielding the Soul Cube on Argent D'Nur. Likewise Doom Slayer is also found in a very similar looking sarcophagus (with a very similar backstory, of being an 'ancient warrior' who fought through hell, a champion who saved mars, and other dimensions across time and space).

The reference to the mother who birthed all demons is a reference to Doom 64, which is also reused in Doom 2016 for the backstory for Doom Slayer, and apears is tied together in The Lost Levels and the return of the Unmaker.

The name of Olivia Pierce who has the Soul Cube in Doom (2016) appears to be studying it and sarcophagus appears to be a reference to Pierce Rogers, as both share same name "Pierce".

The title "The Hero or Martian Hero" is likely a nod back to the Doom and Doom II manuals, where the Marine is sometimes referred to as the Hero, or Our Hero, and said to have been stationed on Mars, or a savior of Mars.