Meru is a prince from the underwater world—or more specifically, from Maron, a water planet. He's got amnesia covering basically his entire past: no memories of his family, childhood, or even what his own race is. Growing up this way left him pretty isolated, though his servant Maru stuck by him through it all.
He's betrothed to Natsumi Hinata, who he's convinced is his princess. At some point, the two of them get caught up with a weapon called the Mer Ball (think of it like a water-world version of the Kero Ball) that's tied to their titles in the kingdom. When things go sideways, Kululu spills the real story: Meru and Maru are actually from Maron, and Meru's destiny was to flood Pekopon and turn it into his home world. The Keroro Platoon saved both of them, along with Fuyuki and Momoka, before that could happen.
Once the corrupted Mer Ball is taken out of play, Meru and Maru decide they're done with their old lives. They end up falling for each other and head back to Maron to start fresh as its prince and princess, saying goodbye to their friends in the process.
Content compiled by AnimeList.moe from publicly available sources.
