Mr. X is a recurring character of Amphibia, serving as a minor antagonist for the first half of the third season. He is a U.S. government special agent from the FBI who arrives with Jenny to Los Angeles trying to capture the Plantars who are with Anne. Although he does eventually capture the Plantars, they are rescued by the friends they made on Earth as well as the Boonchuys and escape to their home dimension, leaving Mr. X with Anne's parents, who proceed to explain what is going on and apparently scold him for automatically assuming the worst about the "frog aliens" and treating their daughter like a criminal.
He returns as an ally during Andrias's invasion of Earth with the backing of the US government with US military to help Anne and her friends fight back against Andrias's invasion force. After the series, Mr. X is shown to have more redeeming qualities, as he willingly provided aid to help Terri create a new interdimensional portal to hopefully help Anne and her friends reunite with those in Amphibia. It's noted that he kept the entire project on the down-low, even to his superiors in the government, as he was wary of the abuse of power such technology could have for Earth, showing a high degree of conscience and morality.