According to the game's official Twitter account, the developers claimed that the only characters they would not add to the game if they became, or were discovered to secretly be, public domain were Calvin and Hobbes from the comic strip of the same name. While the characters are still copyrighted thereby ruling them out, the reasoning for refusing to consider them was out of respect for creator Bill Watterson and his refusal to license the comics for merchandise despite their popularity, feeling that it would go against the strip's ideals and ruin its authenticity.

Felix the Cat, the new playable character announced alongside the Kickstarter campaign for the game's story mode DLC RULE of Inverse Ninjas, was decided through a Twitter poll. The other three characters on the poll were The Continental Op (a private investigator character created by Dashiell Hammett in 1923 whose copyright expired in 2019), Gnorm Gnat (an insect character created by Jim Davis prior to Garfield who entered the public domain as a result of his original comics not having a copyright notice), and Tarcis the Lover (a joke slot seemingly originating from a dating simulator made by this game's co-developer Trainwreck Studios, and that voting for was actively discouraged by the developers).

Inverse Ninjas vs. The Public Domain marks one of the first uses of Mickey Mouse in a non-parody-protected legitimate commercial product for a North American market without Disney endorsement, with Mickey's update being planned to come out to the hour of his entrance into the public domain, but releasing early by mistake. The social media account for the game noted that, although the game's concept is quite silly and lighthearted, utilizing the juxtaposition of characters who would typically not be armed wielding guns, and in spite of the long history of counterculture artists highlighting Mickey Mouse for mockery because of the saccharine image he would develop and how it juxtaposes against Disney's cutthroat corporate activities, it was intended as a sincere tribute to the characters featured, rather than a satire or parody, also noting that Mickey Mouse had been depicted wielding guns in earlier Disney cartoons: