Not a Manmosu Marimo character per se but like Skoo its one character from another older novel by again Nemuneu, it seems to be a fallen hero dominated by a uniform goo, unfortunately like the Skoo ones you will need to learn japanese or use some translator.

First time Marimo did some fan art:
https://twitter.com/marimo1899/status/1310317198619996160

The Novel, Pixiv and Twitter page from Nemuneu:
https://www.pixiv.net/novel/show.php?id=11753324
https://www.pixiv.net/en/users/982931
https://twitter.com/nemuneu