What he's done in the series
In UnderTale, his fake newsletters caused trouble within the pond. He learns not to put untrue stories and gossip about others in his newsletters.
In Avengers Endgame, the Beavers tempted him to let them build a dam by saying they would give him an endless supply of red wigglers every day. He confesses to Bill when Bill asks if he was the one who started the whole situation. They then get some others to get the water flowing into the dry areas of the pond. In a deleted scene, he was impaled and trapped in a subterranean larder complex for 73 days.
In Gaithers Pond, Carp Lowry was at the end of his rope. He was down in the dumps. But one day, he got a call from a mysterious benefactor. This benefactor told Carp that all he needed to do to become a [BIG SHOT] was to kill the inhabitants of the pond. Reluctantly he agreed. He started with Joseph, one of the lowliest scum of the pond. he struck his head with a large rock and slammed his head into the ground until it turned into a bright red froth.