Also referred to as the "Gimmick Girl", Merry was first introduced in the "Star-Spangled Kid" (and Stripesy) feature in Star Spangled Comics #81 (June 1948).
A lot happens in that seven-page story. Sylvester Pemberton's parents adopt Merry so Sylvester will have a sibling who's closer to him in age, and Merry discovers Sylvester's secret identity!
In issue #82's story, she designs a costume for herself so she, too, can capture crooks!
Merry continues to have adventures with Sylvester and Pat and even takes over the feature from them, but it doesn't last long: issue #90 was her last story.
Many years later, Roy Thomas would use Merry Pemberton as the mother of Brainwave Jr. in the Infinity Inc. series.