Natural gates have been opening all over the world throughout history but the world is mostly water so that's where they usually happen. The gates don't stay open long so a group of merfolk got trapped in the upside down for a while and established their own population back when the upside down still had water. After many generations, the gates opened again and allowed the upside down merfolk to hybridize with the right side up merfolk. This cycle of hybridization and isolation has happened for millennia.
Steve is from a line of merfolk that's almost entirely from the right side up but he is aware of the upside down because all merfolk now have low level telepathy and have an extensive oral history conveyed through a combination of song and shared memory.
Also a bit of lore borrowed/expanded from The 13th Year: Young merfolk look entirely human and need to grow up on land. Around 13 years old, they go through their first transformation and can decide from there if they want to stay in human form or merfolk form or if they want to go back and forth. Merfolk parents usually stay on land with their kids until they hit 13.
A looooooooooooong time ago, merfolk used to prey upon the ancestors of humans by luring them into the water through song but they also would do a brood parasite type thing by leaving their babies on shore for the human ancestors to find and take care of; this allowed the baby to better learn their preys vocalizations for later after they returned to the water.
After a whole bunch of evolution, it became clear that humans were getting smarter and more dangerous so they stopped being a worthwhile food source. After a while, some merfolk decided to live with humans instead of going back to the water. Eventually not eating humans became less about hunting them being risky and more of a borderline cannibalism type taboo.
All merfolk have both sets of reproductive organs. The upside down population has generally darker colors. Steve is especially bright even for a right side up merfolk because he's from a population that's from a coral reef. His moles become golden scales in his merfolk form. His parents have decided to permanently go back to the ocean after Steve did his first transformation.
Barb survives because Steve has better senses than a human even in human form so he heard the demogorgan coming and fucking launched himself at it and dragged into the pool and furiously ate it.
And then eventually he kisses Eddie
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