Soooo... I actually have a few friends who practice the modern martial art of Ninjutsu. They are also fans and students of the history of Ninjas. One common misconception about Ninjas is that they had some special love of their peculiar weapons. While, in fact, they are all about efficiency, and using the best tools for the job. So, a modern ninja is just as likely to use an AR as a bow, depending on which is more suited to the job.
Also, you can imagine someone being born able to project fire, but with no special physical gifts. They might want to get special training to be able to compete in the big bad world of supers. Or a mutant might be born into a Ninja family. Or maybe a trained ninja found a magic gem that gives her special powers.
As an example of a "mis-matched" Ninja, I have a Scrapper that is War Mace/Ninja. Her backstory, in brief, is that she was born into an ancient mystical Ninja sect. But, she is quite a bit bigger and more muscular than the average member of the clan. As a little girl, she was required to take Ninjutsu, but she really preferred the satisfying *crunch* of a heavier weapon, and preferred toe-to-toe fighting over stealth. So, when she was a teenager, she learned mace-wielding from Youtube. On one of her jobs, she "acquired" a magic mace from a target, and boom, mace-swinging ninja! (She's also a lot of fun to play, turns out to be a decently synergistic combo).