The Fawcett comics hero, Mr Scarlet, is Brian Butler, a District Attorney. As a DA he can’t put all of the crooks and criminals away, so at night he dresses up in a bright red costume and goes out to kick crook butts. The Public Domain superheroes website calls him a “vigilante,” which, to Brian, is okay. If he goes around the law, well then, why not? He’d probably only get bounced from office if the voters knew. Or maybe not. In the era Mr Scarlet and Pinky appeared the idea of a vigilante wasn’t as shady as it is now.
Mr Scarlet (and Pinky) were around for a few years as secondary characters, never getting their own comic book, unlike several other Fawcett heroes.
What surprised me is that writer France Herron and artist Jack Kirby are given credit for creating Mr Scarlet. I should not be surprised, because I have always felt that Jack Kirby could come up with a superhero at the snap of his fast-drawing fingers. But in my opinion, Mr Scarlet seems mostly from hunger, derived from other superheroes.
From Wow Comics #10 (1943). No writer or artist is credited by the Grand Comics Database.