The Axe-bass is an heirloom of Marshall Lee's family. It was originally a battle axe before Marshall Lee converted it into a bass guitar, which Hana Abadeer calls "some kind of lute." When Fionna summons his mother, Hana seems surprised that Marshall Lee has it and takes it from him. In his efforts to reclaim it, he is shown to care somewhat more for the bass than he does for Fionna, as he chooses to save it from hitting the ground rather than Fionna.
It is an entirely red, double-sided battle axe with two chips on the right side. The head stock was most likely added by Marshall Lee, along with the machine heads, two on each side. The pickups, also added by Marshall Lee, appear to be single coils of unknown manufacture. The body has just two knobs (for volume and tone).