Agent X is a one-time character from The Mask: The Animated Series, who appeared only in the season one, Martian Mask. He's a high-ranking FBI agent who believes The Mask is an alien plotting to destroy the world. He is determined to capture and destroy The Mask until the end of the episode when The Mask captures the villains, Putty Thing and Fish Guy and tells the agent he's an Intergalactic Security Agent sent to capture the fugitives.
He was voiced by series regular, Jim Cummings in a gruff voice.
After escaping custody from Lt. Kellaway and Doyle via space rocket, Agent X and the FBI spotted The Mask's rocket from their computers and believes he's an alien. Agent X and his scientist, Crosby, sent out to capture The Mask from Stanley's apartment but Lt. Kellaway and Doyle interfered in their attempts to find out that Stanley is The Mask and arrest him, causing a feud between Kellaway and Agent X in saying that The Mask is a criminal or an alien.
The FBI captured The Mask through attack helicopters with force field guns when trying to capture Putty Thing and Fish Guy after they escaped prison. They placed him in a containment truck to experimented on by Agent X and Crosby. After pulling a series of hilarious jokes around the containment chamber, Lt. Kellaway and Doyle arrived with additional police officers to capture The Mask themselves, causing a belief fight between Kellaway and Agent X while The Mask escaped (easily) and ran off to capture Putty Thing and Fish Guy.
After The Mask captured the runaway villains, the police and FBI arrived to capture The Mask while Lt. Kellaway and Agent X continue to bicker over him being an alien or a criminal. To prove his innocence, The Mask appears from a large flying saucer dressed as an alien and tells police and FBI agents he's an intergalactic security agent sent to capture Putty Thing and Fish Guy and bring them to custody. Kellaway, obviously didn't believe the ploy, but Agent X did and apologizes for the tests he performed on him. The Mask gave Agent X "The Twister", (twisted his nipple), as an act of forgiveness, which an amused Kellaway grinned at. The Mask left in his flying saucer saying goodbye with The X Files quote, "And remember to keep watching the skies." where Agent X cries as he realizes that alien life exists, (including good ones.) which still annoys Kellaway.
- Agent X's appearance bares similarity to Lt. Kellaway, which The Mask pointed out in Martian Mask. His face and hair resembles Kellaway except Agent X's short stature.