The flamethrower was to be inserted in the full game, but for an unknown reasons is not present.
It appears that the weapon is crafted with a water gun (presumably loaded with gasoline) with a wick placed in front of the barrel. Obviously the ignition of the wick would have ignited the flammable liquid fired to obtain a jet of flame.
Presumably it was remarkably effective weapon against enemies.
The flamethrower will be recovered in the locker room of the gym in Leafmore High in the chapter "Imminent Care". It shoot waves of fire that burn the enemies slowly damaging them, the fire ensures continued to cause a real fire. The loader supports 10 shots, plus a maximum of 20 reserve. Despite being extraordinarily effective weapon the charging time is extremely long making it necessary only to reload when you are sure not to be hit by the enemies.