The Drill Sergeant, is a gruff, stern drill sergeant who controls the Smile Away Reformatory School and its prisoners. He 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 Candace finally succeeds 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."
Rescuing Phineas and Ferb
When Candace succeeded in rescuing Phineas and Ferb from the Smile Away Reformatory School, Sergeant Smile-away and his guards give chase. Sergeant Smile-away corners them on a cliff, but just as he is about to capture Phineas and Ferb, Perry and Dr. Doofenshmirtz appear atop the Giant Robot Spider-inator; Doofenshmirtz is defeated and the crumbling of his robot sends Sergeant Smile-away tumbling from the cliff to his (apparent) death.
It is later revealed that the busting of Phineas and Ferb and all subsequent events were a part of a dream sequence in Candace's mind. In the end, Candace's dream was all Perry's dream. Since he only appeared in this dream, whether he exists in real life is unknown.
This character may be parody of Sergeant Hartman in the Stanley Kubrick movie Full Metal Jacket.