Continuity mistake: In the step brother's bedroom, while Brennan is singing "Let's give 'em something to talk about" there is a Samurai sword on the floor in one shot. When they cut back to the same shot the sword is gone.
Continuity mistake: When Dale and Brennan are in the bedroom after their parents just told them that they are selling the house, Dale is sitting on the bed with the mask on. In some of shots, his legs go from being crossed to different positions and sometimes they are flat on the bed. Also, as Brennan sings and Dale is on the side of his bed, his legs change positions.
Continuity mistake: In the first scene of Dale and Brendan climbing on to the gilded lady when it is in the tree, when they're climbing up the ladder and shout "pirate hats!" there's a man visible in the background, but a couple of shots earlier when they walked out of the house there's an overhead shot showing no-one nearby.
Continuity mistake: When Dale and Brennan first meet at the mixer, a lady with a big blue hat keeps appearing and then disappearing in the background.
Plot hole: At the wine mixer, where Dale is working as a chef , there is a band playing on stage. The drum kit used is the exact same drum kit (chimes and all) as Dale's, as seen in the house. If a band was hired to play they would have certainly brought their own equipment, and there is no way Dale would have brought his personal kit to the event, for somebody else to play. Dale ends up playing this drum kit anyway, when the band walks off and Brennan sings.
Continuity mistake: When Brennan is singing at the Catalina wine mixer, and Dale is on the drum set, you notice that the drum beat is not the same sound. It is offbeat. There are more beats than what Dale is producing.
Answer: I doubt it's a person. Jack likely refers to Jack Daniel's, and a handle is usually referred to as a half gallon. Hence the name Jack Handle.