You can read the full theory here.  But in short, when a player is overly resistant to their role or if they are corrupted by an outside source then their role will inverse meaning that their aspect and class unofficially shift to their opposites. Light to Void, Space to Time, Witch to Seer*.

*This can change depending on how you classify the classes into Active/Passive partnerships

There’s an old theory that Eridan’s gills are deformed and don’t work as well as they should and that he wears his scarf to cover them. It would explain why he spends so much time above the water, even having his hive on the surface.

Right

Sollux, Mituna, Rufioh, Possibly the Serkets (we aren’t sure about the vision eightfold), and maybe Eridan if you subscribe to the theory that his gills are deformed

If she went grimdark then why could she speak perfect english?

Vriska

Jade’s inverse is a Seer of Time which she acted like at the beginning of the comic always knowing when things would happen.

(Vriska)’s inverse is Page of Void which she acted like when she mellowed out and Vriska’s piss attitude towards pages is definitely not part of the reason she was so mean to her.

That was obviously Karkat’s opinion since more than one of the trolls is a mutatant

He got it here, when Bro cut it

I’d recommend checking out the wiki page