What exactly even is an essence?
From what I understood so far they're something akin to a heart for the gods.
So, the original text uses the word esencia in Spanish, which translates pretty directly to essence.
An essence is a being's metaphorical heart. It is not their actual heart--as the Symbols don't have actual hearts that pump blood and keep them alive--but it's what makes them a unique individual.
As we see throughout the game, the characters have different essence colors.
Our essence is red.
So is Quinque's.
Duae and Unum's are golden.
Shams, Noor, and Quattor's are blue.
Nulla's is purple.
But Nulla is a little more complicated because he's a conglomeration of several entities (he's layered like an onion!!!)
So it looks like, when his human shell is injured, he bleeds red like a normal human would:
Because Septem, when sharing a human body with Rafael, also bleeds red:
And Nulla can also bleed void, maybe just when he's using it to shapeshift like he was in Quinque's temple?
Anyway, essences are very important because they are what can destroy a being if they're eaten or broken, as we can see from the "GOOD END: ???" in Unum's route.
Because if you destroy the idea--the concept--of who someone "is," that's essentially erasing their existence. Or "killing" them. But you can also "eat" or "absorb" them which is what Nulla does in his route--
...or "transfuse" them, which is how Septem is able to inhabit Rafael.
Note: They can also change color! For example, after his wives are killed, Unum's changes to red due to extreme distress.
(Left: Unum, Right: Quinque)
Losing them changed who he was at his core.
So, essence, heart, core, sense of self...whatever you want to call it, it's wrapped up in this little glowing orb.
