If you look up Shadow Lass' creators, you will invariably find Jim Shooter and Curt Swan listed, as they were the creative team of Adventure Comics #365 (along with inker George Klein), which contains her first appearance. In fact, those creators were also the creative team of Adventure Comics #354, which features the adult Legionnaires, and shows a statue of a deceased Legionnaire named Shadow WOMAN...
who clearly is meant to be the same person when Shadow Lass shows up in #365 (wearing the same costume - although with different color skin. In #354, her skin is white, in #365, it is blue).
However, Shadow Lass was actually created by two fans!
You see, for a longtime in the 1960s, Legion of Super-Heroes Editor Mort Weisinger encouraged fans to write in to The Legion Outpost (the letter column of Adventure Comics) with their ideas for new Legionnaires. Occasionally these ideas would be used on background characters and even, notably, one Legion of Substitute Heroes member, Color Kid, who was created by fan Jeff Greenberg.
But for the most part, this was just a way to make the fans feel involved. Assistant Editor (and letter column head) E. Nelson Bridwell would occasionally post fan suggestions in a "Bits of Legion Business" section of the letter column.
Here's a sample one from Adventure Comics #312...
And here's the one from #342, crediting Greenberg...
But Shadow Lass' creation was not mentioned in any issue (which is why Shooter and Swan still get credit for her creation).
What happened was that Legion fans and would-be comic book writers George Vincent and Mike Rickford came up with two characters that they thought would be good Legionnaires, Chemical King and Shadow Lass.
Both characters ended up being used in the Adult Legionnaire issue.
(Chemical King's would later be shown in Adventure Comics #371).
However, Vincent and Rickford had a much different origin in mind for Shadow Lass. She was intended to be black, from an all-black planet colonized from black people from Earth who left Earth due to racial strife.
Weisinger thought that the idea was way too controversial, but he said he liked the character and would try to work her in in a different fashion.
And notably, in the letter column to Adventure Comics #363, two issues before Shadow Lass made her first appearance, a reader asks about "Negroes" in the comic, and the response sure seems to indicate that a blue-skinned alien was meant to be somewhat analogous (in her early appearances, Shadow Lass' skin color was a notable plot point).
Shadow Lass has gone on to become a major member of the Legion, so congrats to Misters Vincent and Rickford (and heck, Color Kid has become somewhat notable over the years, too, so kudos to Mister Greenberg, as well!).