He first appears in the episode "All Your Fault" as a giant face who inquires whom Finn and Jake are and then leads them to the dungeon holding the Lemongrab earls. He ends up rising to his full size and potential when ordered by the Lemongrabs to lay siege on the Candy Kingdom, hoping to use their candy to make even more of their own "creations" and to feed the rest of their already made lemon creatures. In order to save the kingdom, Finn and Jake try to attack his heart, but instead of killing him, they end up awakening feelings the creature never felt prior to that moment.
Whoa, hold the phone. What is this powerful new juice coursing fromst my core source? Aughhh! The juice aches. Is this the rumored ache of feeling? The feeling of caring unknown to lemons? New thoughts emerge... If I act, the candy people will suffer; if I don't, the lemon people will suffer... The greater good demands but one course only! That I dissolve the bonds uniting me and become component to all!
Upon completing this, Lemonjon explodes, turning into a pile of lemon candies so the earls and Lemon Children may have food, leaving the Candy Kingdom unharmed.
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.
- The lemon candies resulting from Lemonjon's self destruction resemble Lemonheads, a real-world lemon-flavored brand of candy.
- Lemonjon is the only Lemon child able to talk before "Too Old."