Mervin M. Mouser is the head of the Spinachovian Secret Service and the kingdom's sole detective.
Creation and development in Thimble Theatre
Six months prior to his encounter with Popeye and Swee'Pea, Mr. Mouser was assigned by King Blozo to retrieve the map leading to the country's royal jewels, which was stolen by the notorious criminals Samuel Snagg and Baby Doll. It was then up to Mouser to find and retrieve the map and arrest the scoundrels before it's too late.
Six months had passed since Mervin went in search of the crooks who had mysteriously disappeared, but unbeknownst to him they were now for unknown reasons going after Popeye's adopted ward Swee'Pea. Just as Popeye began growing concerned over the would-be kidnappers, wondering what they were after in Swee'Pea, a large package suddenly arrived and inside was Mervin M. Mouser who wished to speak with Popeye in hopes of informing him of what transpired in Spinachovia six months ago. While they spoke, Swee'Pea went back outside, only to run into Snagg again, but having had enough of his kidnappers, Swee'Pea was quick to let them have it with his own spinach empowered might, forcing Snagg to retreat once more. As Swee'Pea went back inside, Mouser had already explained all the details to Popeye and even showed him a picture of the suspected thieves, and their mugs were eventually recognized by Swee'Pea who identified them as being the same individuals who were trying to kidnap him.
Meanwhile in the front yard, Snagg and Baby Doll tried a new scheme by luring Swee'Pea into a pit trap as soon as he walked out the front door after knocking which would secure him inside a leather sack. As soon as the front door opened to answer the knock, their supposed intended target fell in the pit trap's bag and they quickly ran off with him back to their hideout, but when they made it back, it was actually Popeye who had fallen in and quickly gave the two brutes the beating of their lives and took them back to his home for Mr. Mouser to apprehend. Once caught and threatened by Popeye and Swee'Pea, the now cowardly duo revealed what they had done to Swee'Pea's back six months ago, and the invisible ink on his back was made visible after ingesting some warm water as the crooks had instructed, thus securing the location of the "map" as being none other than Swee'Pea himself. With the crooks arrested and the royal jewels safe, Mouser went on his way, although Popeye was forced to pay for his delivery back to Spinachovia.