Superman is a comic book superhero who has appeared in DC Comic books since June, 1938. Created by Jerry Siegel and Joe Shuster in 1932, the concept was sold to Detective Comics, Inc. Since 1938 Superman has appeared in comics, radio serials, television programs, films, cartoons and video games. The character's appearance is distinctive and iconic: a blue, red and yellow costume, complete with cape a stylized "S" on his chest.
Superman was born Kal-El before being rocketed to as an infant by his scientist father Jor-El moments before the home planet Krypton was destroyed. Discovered and adopted by the Kents, the child is raised as Clark Kent in Smallville and imbued with a strong moral compass. Very early he started to display superpowers, which upon reaching maturity he resolved to use for the benefit of humanity.
Superman is a founding member of the Justice League of America.
Superman is Leonard's favorite superhero and Superman's birth name Kal-El is Leonard's most common computer password. He also wears "Clark Kent"-style glasses.
In "The Big Bran Hypothesis" (S01E02), it is revealed that Leonard has 2,600+ Superman comic books.
In The Hamburger Postulate (S01E05), the gang is discussing Superman powers and that he is vulnerable to magic.
In "The Cooper-Hofstadter Polarization" (S01E09), the guys were planning a Superman movie marathon and start discussing Superman with Penny. She says that she remembers Superman catching Lois Lane after she falls off the Daily Planet building. Sheldon tells her that she was travelling so fast that when Superman's arms of steel caught her, she would be sliced into three equal pieces.
In "The Bath Item Gift Hypothesis" (S02E11), they discuss what Superman does when he sweats. The scenario includes him visiting the bottled city of Kandor and engaging in a game of Kryptonian tether ball. He then has to find a Kandorian dry cleaner before he returns to Earth.
In "The Vegas Renormalization" (S02E21), Sheldon plans on a quiet weekend in his Fortress of Solitude when Leonard and the guys are spending the weekend in Las Vegas. Penny refers to his apartment as Superman's big ice thingy which Sheldon gets locked out of.
In "The Justice League Recombination" (S04E11), Zack Johnson dresses up as Superman replacing the smaller less muscular Leonard for their Justice League of America costume team to win the Comic Book Store New Year's Eve contest. Zack enters the apartment and can't remember the "It's a bird..." opening. They win the contest.
In "The Good Guy Fluctuation" (S05E07), Alice, a comic book store customer, asks if she can flirt with Leonard and distract him to get a rare comic book. Leonard says that she could, but she would be using her superpowers for evil. She replies referring to Superman: I'm forbidden by my Kryptonian father to do so. On the side, the actress' husband Brandon Routh played Superman in the movie "Superman Returns".