Hades: 14 Awesome Details You Totally Missed
3. Heracles
Heracles (or Hercules, his more popular Roman name) pops up very often throughout the game, despite the figure himself not actually being present.
When Zagreus first hears of a Champion in Elysium, he immediately assumes it's Heracles, even causing Theseus to feel jealousy (which he vehemently denies) over it. There are "Nemean Chariots" in Elysium which reference the Nemean Lion Heracles slew in life.
Hades himself speaks ill of Heracles, alluding to his awful acts in life (such as the murder of his own family), and also possibly the way he was able to escape punishment by being given a seat on honour in Olympus.
There's even a late game decor choice where you can put up an enormous statue of Heracles in the House of Hades, which Achilles slyly comments isn't the worst thing to stare at while he's on guard duty. Achilles himself mentions he met the hero during his brief stint in Tartarus, the contrast made between Heracles's strength and Achilles's speed, despite their similar demigod bloodlines.