Concept: Mute!Eridan
He knows a bit of sign language but he gets way too confused sometimes and usually just gestures, writes it down, or, if the situation allows, he’ll message whoever he’s talking to on Trollian. Whatever’s easiest/most convenient.
He can laugh/cry/grunt/etc. He just can’t make any verbal noises.
It frustrates him really badly, not being able to communicate “normally” (his thoughts, not my own actual views).
He’s had more than one night where he’s broken down sobbing talking to Feferi because of it.
He was born mute.
Feferi and Vriska were the only ones who knew for the longest time because none of the others had ever talked to Eridan on anything besides Trollian.
He tried to pretend he just didn’t like talking.
The only reason they figured it out so quickly is because Tavros, upon noticing that the violet had so far only nodded or made small noises, said “Can you talk?”
Eridan flinching basically answered it for them.
(Feel free to add on if you want to)
