Neptr (or N.E.P.T.R., an acronym for Never Ending Pie Throwing Robot) is a robot Finn created solely to prank Jake who first appears in "What is Life?." He is a pie-throwing robot Finn created and brought to life so he could prank Jake. He usually calls Finn "Creator". He came to life when Finn became frustrated that his 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. Finn took him to Ice King's palace to use the villain's lightning bolts to fully charge him. When he was hit with Ice King's lightning bolt during the resulting battle, part of Neptr's personality was changed to a desire to collect princesses causing him to become slightly evil, but he also becomes powered up to full power. Neptr admires Finn's pranks and has decided to keep Finn as his companion, and has also decided to prank Ice King hardcore. However, he is shown to have some compassion, as he says goodbye to the (unconscious) Ice King before leaving with Finn, causing Ice King to shed a tear. After this, he and Finn throw never-ending pies at Jake.
Neptr returns in "Hot To The Touch," where it is revealed that he was hiding for 15 months, 4 days, and 9 hours in a game of Hide and Seek and that was accidently forgetten by Finn and Jake. He assists the two by constructing (crudely made) heat-retardant suits in order to get close to the Flame Princess.
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.
Trivia In the end credits for "What is Life?," Neptr is listed in the credits as "N.E.P.T.E.R.," though it is unknown as to whether this was his intended name, or if the "E" was included as a mistake.
Adam Muto stated that technically Neptr is half prince because Ice King unlocked his full potential, which so far makes him the only prince/half-prince in Adventure Time that isn't imagined.
It is unknown where Neptr gets the ingredients for the pies he makes, possibly generating them spontaneously through magic rather than machinery. Neptr is like the monster from Mary Shelley's Frankenstein as both are brought to life by lightning and are made from salvaged non-living parts. In a sneak peek of "Hot to the Touch," Neptr's name was spelled "Neptor," in Latin America his name is Neptor. It also shows Neptr's first song, "Working for the Master," and the fact that he has been hiding for 15 months, 4 days, and 9 hours. Neptr has been the character that has had the most episodes without appearing in Adventure Time as he hadn't appeared in 64 episodes since his first appearance and his second one.