Alright. Slept on it, and I'm going to throw a few things at the wall regarding the bad guys mentioned in the blurb so far, plus some of the other and seeing if there's anything good here that could be of use setting-wise.

Canonically, Strega (the group's name) were the victims of some extremely messed up experiments as children, thanks to a huge and morally dubious company named the Kirijo Group. While they were granted powers, there were only three survivors, and they're dying due to the drugs they take to control the experiments' side effects.
When Chidori's gone, the leftover members go off the deep end. They start a cult worshipping the game's main villain, Nyx, a being who (short version) exists to bring about the death of humanity.

In this setting, I'm throwing Strega out as the name of their species. Living energy aliens is basically the order of the day. The trio here are again going to be the survivors of Kirijo Group experiments - here, Kirijo is performing experiments that have to do with superpower induction, and the powers and potential of this particular species have made them very interesting to their scientists. On escaping, they are obviously very unhappy with humanity. Earth's conditions (yellow sun? oxygen atmosphere?) are slowly killing them, too. They're stranded, they're lost, they're not afraid of death. The remaining two members of the group could still be doing villain type things out there.

Been discussing things with the player who reserved Mitsuru, and there's talk of the possibility of the Kirijo Group being up to a lot of interesting shenanigans apart from this stuff.

As for Nyx? Well, there's always room for a good planet-destroyer in superhero lore, isn't there?