The villain who is known for saying the following line, "Why so serious?" is The Joker. The Joker is a fictional supervillain created by Bill Finger, Bob Kane, and Jerry Robinson, who appears in American comic books published by DC Comics. The character first appeared in Batman #1 (1940) and has since become one of the most notorious and recognizable villains in all of comic book history. The Joker has been portrayed in various media, including films, television series, and video games, and he is known for his chaotic and unpredictable nature coupled with his signature maniacal laugh.