Although I enjoyed SHOWCASE PRESENTS SUPERGIRL VOL. 1 (cover of debut issue by Curt Swan, all rights with current holder), I think like a lot of “Superman family” books of the Silver Age it’s very much a YMMV recommendation. These books were written for kids and while that didn’t preclude characterization and good story-telling, the books about Supergirl, Jimmy Olsen and Lois Lane were often lacking in both. In one story in the collection, Supergirl worries Superman—her cousin and closest relative—will exile her from Earth forever for making a mistake. In another, an amnesiac Jimmy Olsen (there’s lots of amnesia in this book, either natural, accidental, or induced by Superman or Supergirl for some reason) gets sent to the same orphanage as Supergirl lived at the time, under the assumption he’s a kid even though he’d have been 19 or 20.
Those flaws aside, I did enjoy the collection overall (some stories are just clunkers). After one story where Jimmy Olsen creates a magical supergirl—I’m guessing it sold well enough to prompt the creation of the real deal—we plunge into the adventures of Kara Zor-El, an inhabitant of Argo City, which was torn from Krypton in a solid chunk, protected under her father’s experimental weather dome. When the ground turns to kryptonite, the citizens shield themselves with lead; when meteors penetrate the lead, Zor-El sends his 14-year-old daughter to her cousin on Earth (spotted by Kryptonian high-tech telescopes) while the rest of the city died (this origin was tweaked and adjusted over the years but the core remained the same).
On Earth, Superman sets her up as Linda Lee, an orphan (rather than risk suspicion of his identity by living with him then having Superman’s cousin appear) and keeps her existence a secret until she’s fully trained. Her activities in this volume include helping people in secret, meeting other members of the Superman family (Krypto, Superboy in the past), and trying to avoid being adopted for fear her new parents will learn her secret. Plus working hard to train and to prove to Superman that she’s ready.
It’s the latter part that makes the collection stand out from the other super-books. Unlike Dick Grayson being Bruce’s ward and living with him—a set-up that lasted around 70 years until Dick went to college—Linda living at an orphanage, keeping her existence secret from the world, doesn’t feel like a long-term arrangement. Even less so as it goes on and Supergirl tries harder and harder to prove she’s ready—I wonder if readers were requesting she go public, so the creators kept having to find reasons why she wasn’t up to it yet. As it goes along, it almost feels like a long-term dramatic arc, though I’m sure it wasn’t (not at all the style in those days).
The last few stories in the collection do indeed form an arc though. Just as Superman decides Supergirl is ready, her powers vanish. An ordinary girl, she lets herself get adopted and gets a boyfriend (Dick Malverne, who’d turned up in an earlier orphanage story). She doesn’t know the power loss is the work of Kandorian scientist and Kara lookalike Lesla-Lar, who then trades places with Kara, programming her brain to think she’s Lesla. Lesla then tells Superman her powers have come back, makes a secret deal with Luthor—but at the end she’s captured by the Kandorians and imprisoned. Although perfectly poised to be Supergirl’s arch-foe that it’s surprising she only appeared once more in the Silver Age, getting disintegrated by a Phantom Zone villain, and then in a few issues of Superman Family more than 14 years later.
Supergirl returned to her normal super-self a few issues later, then finally got to make her public debut, though regrettably that’s not in this collection but Vol. 2.