Mr. Nimbus is Rick’s ocean-ruling nemesis introduced in “Mort Dinner Rick Andre.”
Mr. Nimbus appears after Morty crash-lands Rick’s spacecraft in the ocean while trying to keep a date with Jessica. Rick explains that he and Nimbus have a longstanding conflict, rooted in a treaty requiring Rick not to touch the ocean. Nimbus controls the ocean and regards the landing as a serious violation.
The episode’s central plot involves Rick preparing a diplomatic dinner for Nimbus at the Smith house. Nimbus is not merely a sea creature or local villain: he is presented as the King of the Ocean and ruler of Atlantis, with hydrokinetic abilities and authority over the police.
Viewers sometimes confuse Nimbus with other Rick and Morty antagonists because Rick has many enemies. Gearhead is an ally who betrays Rick, Squanchy is Birdperson’s friend, and Mr. Needful is a separate character from “Something Ricked This Way Comes.” Nimbus is specifically the ocean-based nemesis.