No, I didn't forget about it, you forgot about it.
Anyway, this week's hero is from a slightly older era than the typical hero covered here - all the way back from 1921 (making him public domain by more traditional means). He's also not from a comic and doesn't have a public-domain illustration, hence the watermarked stock image.
Richard Staegal was just your average wealthy socialite until, one day, he made a pact with his fiance to live a more exciting life. Most couples would accomplish this by [think of lewd joke later], but Richard opted to take on a new life as a Robin Hood-type figure, stealing from the rich and giving to charity.
Of course, he couldn't just go out on the streets without a disguise, hence why the Man in Purple is clad in purple clothing and a purple hood - all of which are apparently specially made to dissolve in acid (that sounds like all clothing, but maybe they dissolve quickly or dissolve under weaker acid than most clothing).
Despite being a creation of Johnston McCulley (perhaps best known for creating Zorro) barely anyone cares about this guy apparently. He showed up in 3 stories total (all of which I could only find by trudging through Archive.org - though there is one publisher selling an anthology with all of them) and, for the first time ever, I've had to create the pdsh wiki page for them. So, one could argue I actually wrote this post 4 days ago and therefore I'm actually early rather than late!