Chlorine

Brook is an introverted genderfluid person suffering from depression supported by their Nova sister, because Mom's a jerk and dad died. And then one day they drowned. Oh no!

Siblings as Novas is rare, but nobody really knows whether there's a genetic link determining who gets Nova powers or not. As it turns out, Brook lucked out and managed to be one of these rare instances, as the water they fell in seemed to rise up, and form into the shape of a person. The ability to turn into liquid. Nifty.

Most Novas have this inherent ability called "Attunement." At the lowest level of Attunement, you apply basic levels of immunity to your powers to your clothing, for example. So you can grow giant and not tear anything up, or be on fire without burning your jeans to a crisp. Brook, as it so happens, does not have that. Anything they want to turn into liquid alongside them has to be made out of Eufiber, a substance that uniquely attunes ITSELF to Nova abilities. Despite this, Brook always wears an earring they've had since they were young. It's small enough to fit through most cracks, so they just carry it alongside their liquid form when they need to. It's not an issue, generally.

Brook is pretty cool with the fact that becoming a Nova means getting to take on a different moniker. Nobody ever asks 'but Chlorine is a boy's/girl's name'. They can do whatever there. But, y'know, there's that spiderman thing. Great power, great responsibility. And, well... Brook isn't much for... action. But maybe Chlorine can be. So Chlorine gets their sister to take them along sometimes, to provide support. Chlorine will probably never be the type to take charge, but is glad to be able to help. And for them... that's enough to make them feel a little better about themselves.