Mermando is a character from TV-series Gravity Falls episode "The Deep End" in season 1. Mermando is voiced by Matt Chapman.
He was swimming with his friends, the mighty dolphins, in the Gulf of Mexico, when he was ensnared in a net by an old sea captain and a shipmate.
The captain announced they would feast on his flesh and had a truck take him to the Bait Shop at Lake Gravity Falls. He managed to escape as the truck was parking and jumped into the lake. He tried swimming against the current and up a waterfall but failed when a log crashed into him in the face, in the process losing a front tooth. He ended up flopping onto the ground, but luckily the local wildlife was kind (or hungry) enough, that they kept licking him, keeping him hydrated. Unfortunately, the licks that were keeping him alive were also ticklish and he stumbled into a water drain that led to the Gravity Falls Pool. For an undetermined amount of time, he was trapped there, hiding his secret and unable to get home, until he met Mabel.
On the hottest day of the Summer, Mabel met Mermando on the pool's opening day. Mermando found her charming and forward and took a liking to her. After their first meeting, Mabel asks him out on a date, and that night she meets him at the pool. Mabel attempts to kiss him by puckering up, much to his confusion.
Mabel agrees to help Mermando return to the sea and sneaks into the pool again by taking the Mystery Cart there, where she loaded the back with the Pool's cooler and filled it with water. Mermando gets in and hides, while Dipper attempts to get Mabel to return the cooler. With a little distraction, Mabel and Mermando make their escape to Lake Gravity Falls. Unfortunately the cooler springs a leak, and Mermando, so close to the water, nearly suffocates to death. Mabel convinces Dipper to give Mermando "reverse CPR", which he reluctantly does (though Mermando afterward points out that he could've just rolled him over to the lake, which was just a few steps away).
Now safely returned to the water, Mermando thanks Mabel and Dipper for their help. While bittersweet, Mabel is glad he would be able to return to his family (and gives him the Pool's megaphone as well, to call out to his family). As a farewell gesture and an expression of his own feelings, he gives Mabel her first kiss. Memando is happily reunited with his family but continues to send Mabel many messages in bottles that end up at the Pool.
- Mermando page on Gravity Falls Wiki.