Sophia ZERO is an upgraded model of Sophia 3rd introduced in Blaster Master Zero that can only be obtained by collecting every single upgrade in the game (including all Maps and, for EX Characters and Destroyer Mode, dummy upgrades and Life Ups) and defeating the Underworld Lord at the end of Area 8.
It is typically available exclusively for use in Area 9, but may be brought into the main Underground after beating the game by starting a new file in Unlimited Mode.
Main Cannon
The most drastic improvement over Sophia 3rd comes in Sophia Zero's main armament. Its Crusher Shot has been empowered to deal ~9.3x damage relative to Sophia 3rd's Crusher Shot; in addition almost all charge shot variants of the Crusher Shot are fired without actually charging, most with greatly improved capabilities.
Laser Shot: Instead of a single projectile, Sophia Zero fires a broad hitscan beam for ~0.2 seconds that instantly damages anything between the barrel and the edge of the screen. Only one beam can be on-screen at a time.
Maximum Shot: The crescent-moon shot is visually upgraded to a half-moon shot; however, there are no unique mechanical changes.
Cannon Shot: Blast radius of each shot is roughly doubled, and Sophia Zero does not get pushback from recoil or take damage from being caught in the blast.
Acceleration Blast: Charge time is halved, and cannot be interrupted by damage. A full SP bar is still required to begin charging. Vertical width is also increased slightly.