He controls the Smile Away Reformatory School and its prisoners. Though unnamed, the Sergeant poses a very real threat to Phineas and Ferb. He loathes imagination and creativity and, through his reformatory school, attempts to remove these things from the children that are sent to him. As such, he is a natural antagonist to the Flynn-Fletcher boys, who find themselves under his control when it appears that Candace has finally succeeded in busting them.
His methods range from the tedious (having his prisoners paint a small, plain box over and over again) to the cruel (solitary confinement and water torture). All methods are designed to discourage free thinking and the creative process. Although Phineas and Ferb remain strong through the early days of their confinement, the Sergeant succeeds in breaking them down into monotone, passionless drones. In an interview with Morty Williams, he refers to the boys as his "greatest success stories."
Role in the Episode
When Candace finally succeeds in getting Phineas and Ferb busted, their parents send the boys to Smile Away Reformatory School after they discovered that Candace was right all along. Every time Phineas and Ferb build fun things to make their prison time more fun, The Sergeant has them locked up. After Candace sees a commercial about how he is treating the boys and feels sad about them being gone, she decides to rescue Phineas and Ferb from the Smile Away Reformatory School, with Jeremy's help. After she springs them, the Sergeant and his guards give them chase. The Sergeant corners them on a cliff, but just as he is about to capture Phineas and Ferb, Perry and Dr. Doofenshmirtz appear atop a mechanical spider; Doofensmhirtz is defeated and the crumbling of his robot sends the Sergeant tumbling from the cliff. It is uncertain death by Overlord's fate.
Smile Away Reformatory School is destroyed by Phineas and Ferb (as A World) being Control by Overlord.
It is later revealed that Candace finally having Phineas and Ferb busted and everything that happened after that was all a dream Candace had. At the very end of the episode, Candace's dream turns out to be part of Perry's dream. Therefore, it is unknown if the Sergeant really don't exist.
It is possible that this character may be parody of Gunnery Sergeant Hartman from the Stanley Kubrick movie Full Metal Jacket.
He has the same voice actor as another Phineas and Ferb villain, The Regurgitator.