Elmer is a baby mouse that was found on Mickey Mouse's doorstep in the 1934 Disney short Mickey Plays Papa. A character similar in design and personality appeared a year before in the short Giantland amongst the orphans.
Elmer was left in a basket on Mickey's doorstep by his mother, who apparently poor and wanted him to have a better upbringing. His cries caused Mickey and Pluto to think he was a monster, but found him in the basket and decided to adopt him. Mickey and Pluto attempted to nurture and entertain him, but their attempts do not impress him and only make him cry until a mishap caused Mickey's nose to resemble Jimmy Durante, causing him to burst into laughter.
He is shown to be bratty and hard to please, but he ends up laughing at what he finds very amusing, such as Mickey imitating Jimmy Durante.