*Before the events of Deltarune, Jevil was the court jester of Card Castle and was friends with Seam, the court magician.

They performed acts together for the kings until one day, Jevil came across a strange someone; ever since Jevil met this person, he had descended into insanity.

His growing insanity is what led to Seam banishing Jevil into the dungeon on the kings' orders, but in Jevil's eyes, he was too fast for the kings and built a wall to "trap" the kings and now, he is the only one "free".

Jevil can be found on the ?????? floor of Card Castle, where he tells Kris, Susie and Ralsei that he is lonely being "free" and wishes to free them as well so they could play. He tells them to find Seam to know how to open his cell, and gives them one of a Broken Key. The player then needs to find the remaining two keys and give them to Smith Malius, who will combine them into the Room Key. After that, the player must go back to Jevil's cell and use the key on the prison cell.

After that, a small door will appear. Upon entering the prison cell, Jevil says that he will play with the heroes, and then with the rest of the kingdom once he's done with them. Ralsei asks what type of game they will play; Jevil then tries to attack them with a Devilsknife; however, the team dodges it. Jevil replies with a "simple numbers game", and that if their HP drops to 0, they "lose".

After Susie and Jevil both compare each other to sharks, the fight commences.

Jevil has multiple attacks during his boss fight, most of which involve him using card symbols to damage the party. His battle has five phases.*