The Grumbo is a monster created by Gumbald out of artificial candy: 100% artificial candy mass according to Huntress Wizard—indicating its laboratory origin. Originally named Bananameister, creating the Grumbo was revealed to be a means of gathering intelligence, evidenced by the camera in its den and the cameras set up along its path to and from the Candy Kingdom. Alternatively, any audio recording devices may be a possibility as opposed to or in tandem with cameras, because Gumbald was able to record the name Huntress Wizard had just given it. Upon entering the Grumbo's den, Finn and Huntress Wizard were able to catch him sleeping which may suggest that the Grumbo is unaware of his purpose, capturing and peeling Banana Guards out of instinct or programming instead of deliberately luring Finn to his den. This is further evidenced by the Grumbo's indifference to Finn riding his back as he made his way back to the den, although this could be explained by the possibility that the Grumbo didn't perceive Finn as a threat and ignored him. The Grumbo was slain by Finn at the end of the episode, exploding in a violent manner after Finn removed his sword.
The Grumbo has a similar texture and color to the Banana guards. Although only seen in the dark and under moonlight, he has a slightly darker color compared to Banana guards seen under the same lighting. While categorization into modern taxa is purely coincidental, the Grumbo is heavily inspired by mammalian carnivores ranging from bears to giant sloths. When lying on his back, it becomes more clear that the Grumbo's upper body is much more developed than his lower body. This characteristic is seen again when Grumbo enters a power stance, with his head held high intentionally appearing bigger—the Grumo's arms are clearly longer than his hind legs. However, the Grumbo appears to be far more agile and responsive when quadrupedal and is seen in this stance far more often, indicating that it is not quite yet bipedal. While the giant sloth was more bipedal in nature, its mammalian nose and canines along with dark patches around his eyes indicate clear inspiration. When sprinting and attacking, the Grumbo resembles a bear far more similarly. When enraged, viscous hot chocolate drips from his mouth.
The Grumbo is vicious and aggressive, showing a permanent scowl with its gnarling teeth. It is very destructive and durable despite being based off bananas. A trail of demolished trees and chocolate puddles are clear indications of his presence. Despite its size and aggressiveness, the Grumbo will sneak up on its prey—entering a room of sleeping banana guards without alerting them. The Grumbo's sleep patterns were never revealed in episode, however attacks appear to occur at night suggesting a nocturnal behavior. The Grumbo will suspend itself from the roof of its cave when sleeping, which is similar to how modern sloths sleep.
The Grumbo is able to produce and launch viscous hot chocolate projectiles with high accuracy at its targets. He demonstratively possesses sensitive hearing, being able to react to the sound of Huntress Wizards's arrows whizzing through the air. In addition, the Grumbo possesses remarkable reflexes evidenced by the aforementioned event. The Grumbo is very powerful, which is to be expected of its size, being able to walk through buildings and toss people around with ease. The hot chocolate produced by the Grumbo appears to possess highly disintergrative properties, nearly completely dissolving Finn's rapier and rendering it useless.
- Grumbo's behavior of kidnapping Banana Guards could be explained by his original name, since Bananameister in German directly translates to Banana master.