The Brain Robot, as its name implies, is a robot controlled by an artificial brain. The brain is stored inside a glass dome filled with fluid to protect and preserve it. It also has eyes connected to the brain through nerves. Its appearance may look sorrowful as it wanders around alone in the sewers of Mission 3.
The entire fight resembles the Huge Hermit's, where the boss chases the player from the left. It continually uses its brainpower to send homing electric balls. When crouched, it can either send a wave of electric balls or electrocute the sewer water. Sometimes, the Brain Robot will pick up a large vehicle and toss it.
Getting too close to the Brain Robot will make it attack with its mechanical claws. Doing this five times enrages the Brain Robot represented by it stomping the ground in place. Its walking animation then changes to wildly swinging its arms back and forth greatly reducing mobility space.
The sewers has several boxes that can be used to dodge the electrocuted waters. Destroying these boxes may give out weapon crates.
The player has access to the Protogunner prior to entering the arena. Unlike the Slugs found in the Huge Hermit fight, the Protogunner's Vulcan does not move in reverse. It does have a Vulcan Fix, locking the Vulcan's position by holding down the fire button.
|Brain Robot: The original model built by the Invaders.|
|Mars Brain Robot: The Martians' take on the Invaders' model, it was built by the Professor. It is stronger than the original model, possessing stronger attacks and is completely red. Appears in Metal Slug Attack.|
|Runaway Brain Robot: A Brain Robot on a rampage, due to constant Invader tinkering on the robot. It possesses more armor and stronger attacks. Appears in Metal Slug Attack.|
|Necro Brain Robot: A Brain Robot filled with necrotic organic parts. Functions similarly to the original in-game apart from the energy shots.|
- The Brain Robot's introductory pose somewhat resembles the Chinese Mantis Kung Fu style.
- In Metal Slug Attack, the Brain Robot's antennae loops incorrectly.