From Keroro Land Magazine! About a decade ago, released as a tie-in for the Keroro Super Movie #2: The Deep Sea Princess.
(Has this been uploaded before? Can’t remember.)
Gives some awesome backstory of Planet Keron and the worldbuilding of Mine Yoshizaki’s universe, and additional info about the Maronians from the second movie and Volume 20 of the manga (which is the manganization of the second movie, with lore and the mystery of the Keron Forces playing a bigger role).
Synopsis! (Or at least the gist of it from my limited understanding of Japanese. If someone wants to translate, you’re so very welcome to!)