Zamrud uses to be a kind guy who literally gave up 3/4 of his humanity (just like how he lost his 3/4 mask) to sow vengeance against humanity until his adoptive younger brother stopped him by pushing him to a portal to another world disguised as a forbidden door

Zilch on the other hand is a cosmic entity who is extra smart, likes using disguise and hates aliens themed on vegetables (mostly onions), often called by Google Lens as "Darkiplier knockoff"

And that other guy... Idk he's based on a Malay folklore where there's a king who accidentally eat curry mixed with blood, got obsessed with blood, and eats kids for dinner