Who's This? The Golden Age teenage heroine on page 10 of Who's Who vol.XV.
How you could have heard of her: Young Justice (the series from almost 15 years, not the recent cartoon series) brought her back and made her a part of Old Justice, former Golden Age sidekicks who take an interest in the younger team. Geoff Johns used her in his Black Reign storyline, while Grant Morrison's 7 Soldiers of Victory features a character called Gimmix, apparently Merry's estranged daughter.
Example story: Star Spangled Comics #90 (1949), reprinted in Adventure Comics #416 (1972)
I don't know what makes me more speechless. The very idea of a mule kick on a stick, or wondering where in the cape she kept it. Though captured, Gimmick Guy escapes again with the help of a fake arm he leaves in a policeman's handcuffs. She tracks him down again, at the Golden Grotto Dining Patio (for some reason, I don't think there would be a lot of gold there), but he steals her cape. Oh noes!
DC Heroes RPG anecdote: One of my players in the early 90s played a male version of Merry without knowing it. His character, Jack of All Trades, had several Omni-Gadgets, but the twist was that I had to make up flash cards with each of these gadgets (one per Power and Skill, essentially) and he would pull like 5 of them at random at the start of each game. Those were the "gimmicks" he had access to during that session. I don't think we even thought of Merry as a close parallel back then, though I don't think it would have deterred him. I like heroes who surprise you with new tricks every time you see them.
Who else? I'm gonna stay in the Golden Age for our next target, if you don't mind.