He is light gray, giant, and apparently part of the Chum Bucket itself. One of his hands is a cannon; the other he just uses to smash stuff with.
Role in game
After having remembered where the Krabby Patty secret formula is, SpongeBob happily wakes up and mentions that it was kept in a safe. Plankton comes out to declare his plan to him after knowing the formula's location and proceeds to transform the Chum Bucket into a giant mech.
However, despite Plankton's original plan of using the robot to attack Krusty Krab, when the robot's antennas are raised, it accidentally sets a switch that makes it attack Bikini Bottom instead.
The PlanktoBot shoots out SpongeBob and moves forward. SpongeBob has to keep up with the mech or else it will destroy the platforms or immediately count as a death. When the mech approaches a building in the middle of the road, it will spawn minions to attack SpongeBob.
After clearing out the enemies, PlanktoBot will destroy the building blocking his pathway and proceed. After chasing it for a while, he will shoot two waves of bullets on the floor and slowly drop his hand on wherever SpongeBob is for the last second. Once the hand lands, SpongeBob must hit it to stun the giant mech, giving SpongeBob time to jump on the Gumpad and hit PlanktoBot's mouth.
After hitting the mouth, SpongeBob enters inside the mech. A wave of robots will chase him down. SpongeBob has to destroy three barrier generators before being able to damage the heart-shaped core on the middle.
Once it is damaged enough, SpongeBob will be shot out of the mech and the cycle will repeat.
Like most of the fights in this version, the fight is almost the same as the console versions. However, SpongeBob instead spawns immediately outside the mech instead of being shot out. Also, mini-bosses from previous stages appear to interfere with Wee Yellow Bot appearing before going inside for the last time and Demento Bot appearing in the mech at the last wave. The only minor difference is the boss' lack of sound effects with the exception the cannon, which uses actual gun sounds.