When Mickey entered the public domain last year, Jenkins did an excellent deep dive into which aspects of Mickey's character and design emerged when:
https://web.law.duke.edu/cspd/mickey/
Jenkins uses this year's entry of Tintin and Popeye into the public domain to further explore the subject of proprietary characters.
Even though copyright extends to characters, it only covers the "copyrightable" parts of those characters.
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