The ax bass is an heirloom of Marceline's family. It was originally a battle ax before Marceline converted it into a bass guitar, which Hunson Abadeer calls "some kind of lute." It makes its debut in "Henchman," where Marceline sings the "Fisherman Song" for the Peanut Baby. Later, she orders Finn to kill the Dimple Plant Monster with it, but he refuses. In the episode "It Came from the Nightosphere," she plays the "Fry Song" with the bass. When Finn summons her father, Marceline's dad seems surprised that Marceline has it and takes it from her. In her efforts to reclaim it, she is shown to care somewhat more for the bass than she does for Finn, as she chooses to save it from hitting the ground rather than Finn.
It shows up in "Go With Me," where she was about to play it, until Finn interrupted her by his singing outside her house. It appears again in "What Was Missing" when she plays it during the songs "I'm Just Your Problem" and "My Best Friends in the World." It makes an appearance in the episode "Marceline's Closet," where Marceline plays it in "Journal Song." Its latest sighting was in the episode "I Remember You," Marceline is briefly seen strumming on it before the Ice King enters her cave, she later plays it during the second half of "Remember You." Marshall Lee has an ax-guitar, which appeared in "Bad Little Boy." He used it to accompany the song "Good Little Girl."
It is an entirely red (except in "Henchman" the edges are silver), double-sided battle ax with two chips on the right side. The head stock was most likely added by Marceline, along with the machine heads, two on each side. The fingerboard has 13 frets. The pickups, also added by Marceline, appear to be single coils of unknown manufacture. The body has just two knobs (for volume and tone).
Marshall Lee is shown playing a variation of the ax bass in the intro to Fionna and Cake episodes; however in "Bad Little Boy," he plays what may be an Ax-guitar, as it has 6 tuning knobs for 6 strings. His Ax-guitar has a head stock shaped like a skull and one blade.
- In "Henchman," the bladed ends were silver; however, in "It Came from the Nightosphere," they were red.
- In "It Came from the Nightosphere," the ax bass is referred to as both a bass and a guitar.
- Marceline's ax bass is highly similar to The Bloo Superdude's guitar in Foster's Home for Imaginary Friends' "The Bloo Superdude and the Magic Potato."
- Somehow the ax bass can be clearly heard in "It Came from the Nightosphere" when Marceline sings the "Fry Song," even though it isn't plugged into an amplifier. This can also be observed in later episodes including "What Was Missing." This is most likely because the bass is enchanted.
- It is unknown whether the ax bass has a truss rod, which curves the neck so that the strings do not lay flat against the fingerboard, although the neck must be curved somehow for Marceline to be able to play it.
- It is one of four basses that Marceline has played, the other three being the Fender Precision Bass in "Evicted!" the Butterfly Bass in "Heat Signature" and the Devil Monster Bass in "Five More Short Graybles."
- The gender swapped version has a different appearance. It is also a guitar in that version, not a bass.
- In the book 'Seeing Red', Marceline states that the battle axe was made into a bass guitar by Hunson Abadeer so she is able to use it.