Fistleo is a humanoid Psuedodroid characterized by his motif with fire and his lion-esque appearance. He has a scarlet, metallic body with green lights on the upper part of it, a mane which can burst into literal fire, and pale metal claws. His forehead has a shard of the Biometal F in his head, which grants him his unique fire powers (which he combines with a fighting style he calls "Reiohken/Reiouken").
Fierce in personality, Fistleo's only interest is the pursuit of power. He hates those he deems weak, sparing hostages only due to the Game of Destiny, and joined Serpent's ranks to elevate his own rank. He trains to sharpen his skills, although he will disrupt it if on orders from Serpent.
After Serpent came to Fistleo before the plot with the offer of grand power, Fistleo transformed himself into a Psuedodroid and willingly took in part of the Biometal Model F's power, with the other half shared with Serpent's other fire general, Flammole. Fistleo leads an assault on Area G on Serpent's orders, lighting a good portion of it on fire and taking several others hostage. Once Aile/Vent confronts him, Fistleo announces his disappointment that the owner of Biometal Z is still young. Regardless, Fistleo attacks them and attempts to destroy them so that they don't interfere in Serpent Inc's business, but is destroyed. His half of Model F is confiscated, and Fistleo is later resurrected briefly to defend Slither Inc. during the final confrontation. After he is defeated for a final time, Fistleo accepts his defeat in grace and believes that after fusing with Model W, he will ascend to the power he always wanted. With that, Fistleo explodes, and ultimately dies for good with Model W's destruction.