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)
The Joker debuted in "Batman" #1 (Spring 1940) as the eponymous character's first villain, shortly after Batman's debut in "Detective Comics" #27 (May 1939). The Joker initially appeared as a remorseless serial killer, modeled after a joker playing card with a mirthless grin, who killed his victims with "Joker venom": a toxin which left their faces smiling grotesquely. The character was intended to be killed in his second appearance in "Batman" #1, after being stabbed in the heart. Finger wanted the Joker to die because of his concern that recurring villains would make Batman appear inept, but was overruled by then-editor Whitney Ellsworth; a hastily drawn panel, indicating that the Joker was still alive, was added to the comic. The Joker went on to appear in nine of "Batman"s first twelve issues.