Yellow (AU: Marvel)

Rin is an inhuman who underwent terrigenesis and came out the other end with the ability to manipulate matter. To put things simply, they became able to alter the physical appearance of anything they pleased- though it isn't as good a gift as it sounds, and takes a lot of trial and error for them to be able to make something look like what she wants. As a result, they have chosen not to use their ability on anything living- including plants and animals- out of fear of the harm they could do.

Unbeknownst to them, their decision to not allow change to come to living creatures is causing a manifestation of their powers to halt appearance changes in anything living around her. In other words, they are subconsciously preventing any living thing around her from ageing.

After developing her powers, Rin chose to become a vigilante hero and strike down against evil in their neighbourhood! They became the masked hero, Star Boy. They chose to present as a boy in their hero form partly due to wanting to be validated as a boy sometimes after presenting as a girl in their civilian life, and partly because they figured that people would be less likely to uncover their identity if they appeared to be two different genders.

As Star Boy, they have a few reoccuring enemies- archenemies, maybe even. Fairly frequently Star Boy has to prevent them from trying to take over the planet... or so he thinks. In reality, these so-called enemies are just petty criminals who are trying to take up his time so he doesn't end up putting himself in true danger by trying to stop a truly cold-hearted criminal. They affectionately call him 'Scarf Boy'.