Question: What fictional superhero created by Jerry Siegel and Joe Shuster was episode fifteen, "Solitude" of the series "Supergirl" named after?

Context paragraphs:
Superman: Superman is a fictional superhero appearing in American comic books published by DC Comics.  The character was created by writer Jerry Siegel and artist Joe Shuster, high school students living in Cleveland, Ohio, in 1933.  They sold Superman to Detective Comics, the future DC Comics, in 1938.  Superman debuted in "Action Comics" #1 (cover-dated June 1938) and subsequently appeared in various radio serials, newspaper strips, television programs, films, and video games.  With this success, Superman helped to create the superhero archetype and establish its primacy within the American comic book.  The character is also referred to by such epithets as the Big Blue Boy Scout, the Man of Steel, the Man of Tomorrow, and the Last Son of Krypton.

Solitude (Supergirl): "Solitude" is the fifteenth episode in the first season of the CBS television series "Supergirl", which aired on February 29, 2016.  The episode's teleplay was written by Anna Musky-Goldwyn and James DeWille, from a story by Rachek Shukert, and directed by Dermott Downs.  The episode is named for the Superman's Fortress of Solitude in comic books and related television and film series.

Superboy (Kal-El): Superboy is a fictional superhero that appears in American comic books published by DC Comics.  The character was created by Jerry Siegel and Don Cameron and is based on the character of Superman that Siegel co-created with Joe Shuster.  Superboy first appeared in the comic book "More Fun Comics #101" in 1945.

Lex Luthor (Smallville): Lex Luthor is a fictional character from the television series "Smallville".  He was a series regular from the pilot episode until the season seven finale, and has been played continuously by Michael Rosenbaum, with various actors portraying the character as a child throughout the series.  The character of Lex Luthor, first created for comic books by Jerry Siegel and Joe Shuster in 1940 as nemesis of Superman, was adapted to television in 2001 by Alfred Gough and Miles Millar—this is only the third time the character has been adapted to a live action television series.  The character has also appeared in various literature based on the "Smallville" television series, none of which directly continues from or into the television episodes.

Lois Lane: Lois Lane is a fictional character appearing in American comic books published by DC Comics.  Created by writer Jerry Siegel and artist Joe Shuster, she first appeared in "Action Comics" #1 (June 1938).  Lois is a reporter for the Metropolis newspaper, the "Daily Planet", an award-winning journalist and the primary love interest of the superhero Superman.

List of Supergirl characters: "Supergirl" is an American television series developed by Greg Berlanti, Ali Adler, and Andrew Kreisberg based on the characters created by Jerry Siegel and Joe Shuster in the "Superman" franchise and Al Plastino and Otto Binder's character Supergirl (Kara Zor-El).  The series stars Melissa Benoist in the titular role of Kara Zor-El / Kara Danvers / Supergirl, as well as Mehcad Brooks, Chyler Leigh, Jeremy Jordan, David Harewood, and Calista Flockhart, with Chris Wood, and Floriana Lima joining in later seasons.  In addition to original characters, several other characters from DC Comics universe also appear throughout the series.  For its first season, "Supergirl" aired on CBS, before moving to The CW for its second season.

Doctor Occult: Doctor Richard Occult is a fictional superhero appearing in American comic books published by DC Comics.  Created by Superman's creators Jerry Siegel and Joe Shuster, Doctor Occult, also known as the Ghost Detective, is a private investigator and user of magic who specializes in cases involving the supernatural.  Doctor Occult is the earliest character created by DC Comics still currently used in the DC Universe.

The Reign of the Superman: "The Reign of the Superman" (January 1933) is a short story written by Jerry Siegel and illustrated by Joe Shuster.  It was the first published use by the writer/artist duo of the name "Superman", which they later applied to their archetypal fictional superhero.  The title character of this story is a telepathic villain rather than a physically powerful hero.  (Although the name is hyphenated between syllables due to it being broken between pages on the story's opening spread, it is spelled "Superman" in the magazine's table of contents and in the story's text.)

Tony Strobl: Anthony Joseph (Tony) Strobl (May 12, 1915 – December 29, 1991) was an American comics artist and animator.  He was born in Cleveland, Ohio and attended Cleveland School of Art from 1933–37, with Jerry Siegel and Joe Shuster, who actually got some help from Strobl creating Superman.  Gerard Jones in his book "Men of Tomorrow" reveals at one point Jerry Siegel contemplated ending his partnership with Joe Shuster in developing what became Superman and work with someone else instead.  Strobl was among those approached but he respectfully declined, feeling his more cartoony artstyle was ill suited for such a serious character.

Clark Kent: Clark Joseph Kent is a fictional character appearing in American comic books published by DC Comics.  Created by Jerry Siegel and Joe Shuster, he debuted in "Action Comics" #1 (June 1938) and serves as the civilian and secret identity of the superhero Superman.

Supporting paragraphs: Solitude (Supergirl) Superman
Answer: Superman