Boy howdy
My gut reaction is to wish for Niji to get kissed by a speeding city bus, but in terms of drama, she's doing a great job of fucking everything up for everyone else.
Kinda impressed at how antagonistic she's being written, when even Meguru managed to turn the corner regarding her trauma and how she was inflicting it on Azuki. Very much one of those "what on Earth happened for you to end up acting like this is okay" sort of deals.
But I think it helps that she's the one who's most unapologetic about her "disgusting love". Remu and Azuki actively hide away their "stuff", and Meguru had past trauma that she's apparently starting to be cognizant about and how it's affecting her behavior and relationships.
Niji's just out here writing (probably questionable-quality) fan fiction while blackmailing her love interest and actively fucking around with the lives of other people to "get back at / penalize" Remu.
She's either uncaring about how she comes across, or wholly ignorant of how problematic she's being, and I'm curious to find out how un/aware she is.