ETYMOLOGY & MYTHOS
Prompto Argentum is latin for Quick Silver, otherwise known as the metal compound, Mercury. Cinnibar (Mercury Sulfide) is used to create the Vermillion pigment, which is a ruddy, reddish color. Red is often a color that Prompto is typically seen wearing. I thought it a little strange that Prompto would just be named soley after a metal element. Normally, Final Fantasy tends to go a little deeper with their naming. This is all on a hunch, but I discovered something interesting while researching "Mercury".
Mercury is also a Roman god. Final Fantasy XV uses a lot of Latin. The names of the characters of course, are derived from Latin words. The god Mercury is often depicted wearing winged shoes and was known for his speed.
The Roman god Mercury is typically seen as the patron of commerce, which really doesn't have too much to do with Prompto outside speed. However, digging deeper, Mercury is also the same diety as Hermes, the greek god.
Hermes is
a god that also is known for his speed, winged shoes and cap. Primarilly, Hemes was the god of boundaries and transitions. He's seen as the messenger for the gods, in addition to being viewed as the protector of theives, herdsmen, oratory and wit, literature,
poetry, the arts, athletics, invention, roads, and boundaries. Hermes was a bit of a trickster in mythology and was known for outwitting the gods for the sake of humanity.
In Homer's Iliad, Hermes is written as an ally of the Greeks against the Trojans. He's called the "bringer of good luck", "excellent in all the tricks" and moreover, "guide and guardian". Lastly, Hermes is known as being a guide and emmisary of souls
to the underworld.
SquareEnix really loves to use mythological references in their games. Mercury/Hermes makes perfect sense in this case. For one, Prompto is quick on his feet. His movements are fast. In Episode Prompto, he has a ton of stamina compared to Noctis. It's interesting to note that out of all activities he could have taken up to lose weight, it was running. He could've gone into resistance training or played any number of sports. He could've even just simply changed up his diet with little physical activity to lose the weight. I don't think his choice of physical activity was an arbitrary storytelling decision. Prompto is excellent at learning technologies and masters Niff machinery quickly and easily, from his various weapons to using a Niff-made flying vehicle to help Noctis reach Leviathan. Prompto is witty, and a photographer which falls under the realm of the arts. Boundaries and transitions really illustrates Prompto perfectly, having been created in Nifelheim and taken to Insomnia in his infancy. Noctis later states at the end of Episode Prompto that the borders between nations be broken down and united, and he doesn't care where he came from. Prompto is one of Noctis' guardians on his journey. Ultimately in the end, leading the king to sacrifice himself to bring back the dawn.

In the original Versus XIII storyline, there was a heavy emphasis upon death, black, and skulls. Some of this imagry and theming made it's way into the final game, but the worship of death was left out. It could be that Prompto originally had a little more to do with this side of Hermes or Mercury's mythology. However, it looks like something that may have been left on the drawing board at this point.
Now, this is all in theory, but the game is riddled with messengers. Umbra and Pryna are both messengers. Umbra being a messenger to the Lucis Caelum family and Pryna to the Nox Flueret line. Gentiana is a messenger of Shiva. It's said that there are dozens more. It could very well be that after Noctis ascended and became the King of kings, in a way perhaps became a god himself. If this were true, Gladio, Ignis, and Prompto thusly would became his messengers.
