Batman
The first Batman story, "The Case of the Chemical Syndicate", was published in "Detective Comics" #27 (May 1939). Finger said, "Batman was originally written in the style of the pulps", and this influence was evident with Batman showing little remorse over killing or maiming criminals. Batman proved a hit character, and he received his own solo title in 1940 while continuing to star in "Detective Comics". By that time, Detective Comics was the top-selling and most influential publisher in the industry; Batman and the company's other major hero, Superman, were the cornerstones of the company's success. The two characters were featured side-by-side as the stars of "World's Finest Comics", which was originally titled "World's Best Comics" when it debuted in fall 1940. Creators including Jerry Robinson and Dick Sprang also worked on the strips during this period.
Joker (character)
DC Comics was a hotbed of experimentation during the 1970s, and in 1975 the character became the first villain to feature as the title character in a comic book series, "The Joker". The series followed the character's interactions with other supervillains, and the first issue was written by O'Neil. Stories balanced between emphasizing the Joker's criminality and making him a likable protagonist whom readers could support. Although he murdered thugs and civilians, he never fought Batman; this made "The Joker" a series in which the character's villainy prevailed over rival villains, instead of a struggle between good and evil. Because the Comics Code Authority mandated punishment for villains, each issue ended with the Joker being apprehended, limiting the scope of each story. The series never found an audience, and "The Joker" was cancelled after nine issues (despite a "next issue" advertisement for an appearance by the Justice League). The complete series became difficult to obtain over time, often commanding high prices from collectors. In 2013, DC Comics reissued the series as a graphic novel.