Neptr builds the fireproof robo suits for Finn and Jake to wear to protect themselves from Flame Princess' fireballs and heat. Neptr creates them from scrap materials, spare parts, and junk he finds in the Tree Fort. Neptr forms the helmet of Finn's suit and shares control with Finn. Finn's suit (unlike Jake's) is shown to have a type of fire extinguisher built into one arm. They are used purely defensively, because Finn does not want to fight Flame Princess.
In "Hot to the Touch," Finn and Jake put on the suits and fly to the Goblin Kingdom, where Flame Princess is burning buildings. Jake uses his suit to put out a flaming cart, apparently crushing it in the process. Aiming at Flame Princess, Neptr readies the extinguisher through a manual overload, but Finn turns the extinguisher away in order to save Flame Princess. Later, Finn pulls open the chest and part of the head of his suit, seemingly damaging Neptr in the process. However, Neptr tells Finn that "Even if everyone burns, you'll still have me," showing that Neptr is not upset and still functional.
The suits are relatively large and are made up of miscellaneous metal objects.
Finn's suit is considerably larger than Jake's. Its left hand is shaped like an oven mitt, and on the back are exhaust pipes. The left and right feet are a small tank and a toilet, respectively, and part of the right leg appears to be a tarnished trophy. It has stickers of a rainbow with clouds on its right arm, a peace symbol on the left shoulder, a unicorn on the back, a kitten on the front left thigh, and a shooting star on the inner left thigh (though some of their positions change from shot to shot).
Jake controls his suit with what looks like a wooden ship's wheel. Its right arm is a backhoe arm and its left leg is a file cabinet mounted on a set of treads. On the top are what appear to be a small jet engine and a miniature MRL. Jake's suit also has stickers of two pink flowers on the front, a black dog with its tongue hanging out on the back of the turbine, and a green snake on the outer left leg.