Neptr (or N.E.P.T.R., an abbreviation for Never Ending Pie Throwing Robot) is a robot Fionna created solely to prank Cake. He is a pie-throwing robot Fionna created and brought to life so he could prank Cake. He usually calls Fionna "creator."
He came to life when Fionna became frustrated that her shoddily-assembled "robot" wasn't working and threw the creation out into the rain where he was hit with a lightning bolt. This brings Neptr as we know him to life, though he isn't fully functional. Fionna took him to Ice Queen's palace to use the villain's lightning bolts to fully charge him. When he was hit with Ice Queen's lightning bolt during the resulting battle, part of Neptr's personality was changed to a desire to collect princes causing him to become slightly evil, but he also becomes powered up to full power. Neptr admires Fionna's pranks and has decided to keep Fionna as her companion, and has also decided to prank Ice Queen hardcore. However, he is shown to have some compassion, as he says goodbye to the (unconscious) Ice Queen before leaving with Fionna, causing Ice Queen to shed a tear. After this, he and Fionna throw never-ending pies at Cake.
Neptr is made from pieces of junk stuck together with tape. His components are an electric oven, a soup can, a circuit-board, wires, a tuning-fork and small tank treads possibly from a toy. Finn drew a face on the soup can, intending this to function as the face, but knobs on Neptr's side became his face when he was brought to life.