The first hidden boss from Chapter 1. Once the court jester of the castle, he went mad after meeting a certain someone and was sealed in the ?????? floor dungeon by Seam, the court magician. You can open his door by obtaining Prison Keys A, B, and C, having Malius the blacksmith repair them, and heading to the cell. Defeating him through Mercy grants Jevil's Tail. Defeating him through Fighting grants the Jevilstail (Devilsknife).

The hidden boss of Chapter 2 (specifically, Spamton NEO). Spamton was once a successful salesman living in a large mansion after meeting someone, but was kicked out after that person disappeared and now lives in a dumpster. To encounter Spamton NEO, you must agree to a deal, buy the KeyGen, use it in Queen's mansion, place Spamton into the Empty Disk found in the abandoned machine in the basement, and then return the disk to the machine. Defeating him through Mercy grants the Dealmaker. Defeating him through Fighting grants the Puppet Scarf.

The hidden boss of Chapter 3. He appears when you move forward after winning the battle against Tenna and obtaining Toriel, following the severing of Tenna's arms. In Chapter 4, he creates six Dark Fountains at once to summon the "Colossus," a being existing only for destruction. However, the fountain inside the Colossus is sealed through the combined strength of the Old Man (Gerson) and Kris's party. Defeating him grants a unique weapon for Noelle and Kris, the Black Shard.

An in-game hidden boss in Chapter 3. If you achieve an S-rank in the game and enter the private room, you can obtain a sword. By leveling this sword to Level 4 and following several steps, you can trigger a battle by entering the bunker area. Defeating him grants the Shadow Cloak.

A hidden boss in Deltarune Chapter 4. In the TV Game Show area of the town below the castle, enter 6453 at the door to Mike's room. After touching the cat statue to make it disappear, a trembling mouse expresses gratitude, and you can battle the 'Real Mouse' by adjusting the volume. He is the only hidden boss who does not give a Shadow Crystal, as he is the easiest in terms of difficulty. His true identity is Green Pippins, Bat-Tat.

A hidden boss in Chapter 4. By collecting sheet music scattered across the map and playing the Golden Piano, a furnace in the library opens. When Susie sees the massive amount of money and asks for another way, he asks for a shave, but dodges every attempt to cut. An annoyed Susie attacks, he dodges, transforms into the Hammer of Justice, and the battle begins. Defeating him grants the Axe of Justice.

A hidden boss in Chapter 5. After collecting 10 Pink Coins across the map and buying a key from the Cliff Shop, you can battle Pink by opening the pink door in the castle. The character is inspired by Mad Mew Mew from the previous work, Undertale. Defeating her does not grant a weapon, but she gives a Shadow Crystal and allows you to buy weapons for the Six Flowers. (As a bonus, Pink kisses Kris on the forehead after receiving the Shadow Crystal.)

A hidden boss in Deltarune Chapter 4. Progresses the same way as Mike and appears after the regular Mike battle. He appears and can only speak in 'meows' (cat language). His true identity is Shadow Guy.

A hidden boss in Deltarune Chapter 4. Progresses the same way as Mike and appears after Flooey (Cat Mike). He is characterized by his use of a regional dialect. His true identity is Zapper.