Lemonjon first appears in the episode "All Your Fault." He is a giant Lemon Person created by the Earls of Lemongrab who is so large that his various organs exist in separate rooms within Lemon Castle. His head appears within the castle itself as a giant living wall with a Lemongrab-esque face.
When Finn and Jake first encounter Lemonjon, he is seen only as a large, rectangular, yellow face. Once he stands up, he reveals that he is a giant, yellow lemon person. He seems to have two noses. He also has fluffy yellow "hair" on the top of his head, divided into three parts.