Answer by taking a quote from the following article:

Robert Dennis Crumb (; born August 30, 1943) is an American cartoonist and musician who often signs his work R. Crumb. His work displays a nostalgia for American folk culture of the late 19th and early 20th centuries, and satire of contemporary American culture. Crumb is a prolific artist and contributed to many of the seminal works of the underground comix movement in the 1960s, including being a founder of the first successful underground comix publication, Zap Comix, contributing to all 16 issues. He was additionally contributing to the East Village Other and many other publications, including a variety of one-off and anthology comics.

Crumb's father gave him $40 when he left home after high school. His first job, in 1962, was drawing novelty greeting cards for American Greetings in Cleveland, Ohio. He stayed with the company for four years, producing hundreds of cards for the company's Hi-Brow line; his superiors had him draw in a cuter style that was to leave a footprint on his work throughout his career. In Cleveland he met a group of young bohemians such as Buzzy Linhart, Liz Johnston, and Harvey Pekar. Dissatisfied with greeting card work, he tried to sell cartoons to comic book companies, who showed little interest in his work. In 1965, cartoonist Harvey Kurtzman printed some of Crumb's work in the humor magazine he edited, Help!. Crumb moved to New York, intending to work with Kurtzman, but Help! ceased publication shortly after. Crumb briefly illustrated bubblegum cards for Topps before returning to Cleveland and American Greetings.  Crumb married Dana Morgan in 1964. Nearly destitute, the couple traveled in Europe, during which Crumb continued to produce work for Kurtzman and American Greetings, and Dana stole food. The relationship was unstable as Crumb frequently went his own way, and he was not close to his son Jesse (b. 1965).  In 1965 and 1966 Crumb had a number of Fritz the Cat strips published in the men's magazine Cavalier. Fritz had appeared in Crumb's work as early as the late 1950s; he was to become a hipster, scam artist, and bohemian until Crumb abandoned the character in 1969.  Crumb was becoming increasingly uncomfortable with his job and marriage when in June 1965 he began taking LSD, a psychedelic drug that was then still legal. He had both good and bad trips. One bad trip left him in a muddled state for half a year, during which for a time he left Dana; the state ended when the two took a strong dose of the drug together in April 1966. Crumb created a number of his best-known characters during his years of LSD use, including Mr. Natural, Angelfood McSpade, and the Snoid.

What were some other characters in his published work?
Crumb briefly illustrated bubblegum cards for Topps before returning to Cleveland and American Greetings.