Mad Dummy is a ghost monster living inside a dummy located in the Garbage Dump. They also serve as a mini-boss in Waterfall.
Foolish. Foolish! FOOLISH!Mad Dummy during the fight.
Mad Dummy takes the appearance of a dummy with a set of upper teeth on the torso, a set of eyes on the head, and nostrils on the nose. However, as Glad Dummy, they instead have two happily closed eyes, along with a smile and blush. Its overworld sprites are orange and red.
Mad Dummy has an aloof attitude when making conclusive presumptions, which is notable when any action the protagonist does to the dummy is considered unfathomable. They also despise the feeling of powerlessness, feeling disarmed when their dummies cannot kill the protagonist quickly. They also act unhinged at exciting points of an event, often repeating three times what they say in a crescendo and uttering maniacal laughter, such as against the protagonist to entice an outward fear of hopelessness and boast their invincibility.
Despite their somewhat psychotic behavior, they can be calm and collected and can cooperate with other people, seeing how they do not bother the protagonist after Napstablook interrupts the battle and serving as Undyne's punching bag for training outside her house.
When Mad Dummy turns into Glad Dummy on the Genocide Route, they become so angry that they became corporeal with their dummy, making them happy and thankful to the protagonist, so if anything, Mad Dummy just needed to let off some steam.
When Mad Dummy is encountered at the Garbage Dump in Waterfall, they reveal that their cousin possessed the dummy in the Ruins.
Mad Dummy's dialogue varies depending on how the protagonist has interacted with the Dummy in the Ruins.
- If the protagonist fled from the Dummy without talking to it, Mad Dummy says that the protagonist broke the Dummy's heart, who was hoping they would talk to them.
- If the protagonist talked to the Dummy, then Mad Dummy says that they said terrible things to the Dummy, spooking the ghost out of their body.
- If the protagonist attacked the Dummy, Mad Dummy says that they (the protagonist) destroyed their home.
- If the protagonist bored the Dummy by repeatedly missing, checking or sparing, Mad Dummy says that the protagonist was really boring and thus their cousin left "like any self-respecting spectre."
Regardless of how the protagonist interacted with the Dummy, Mad Dummy is enraged by it. They attack the protagonist with the intentions of stealing the protagonist's soul. Napstablook later stops the fight. After the fight, they are found outside of Undyne's House taking a job as a punching bag.
True Pacifist Route
When encountering Mad Dummy in the Genocide Route, the amount of hatred directed towards the protagonist's actions allows them to "fuse with their body," fulfilling their lifelong wish. As a reward, they are willing to spare the protagonist as soon as combat begins. Unfortunately for the newly-renamed Glad Dummy, they are now vulnerable to physical attacks.
Regardless of how the protagonist has acted towards the Dummy in the Ruins, Mad Dummy's dialogue is not affected.
- Mad Dummy has several allusions to the JoJo's Bizarre Adventure series, particularly the third part (Stardust Crusaders).
- The music played before the Mad Dummy fight is similar to the track "Apparent Crisis" from the JoJo's Bizarre Adventure: Stardust Crusaders soundtrack, "Destination."
- The sound played during Mad Dummy's introduction scene is similar to Dio Brando's battle intro from JoJo's Bizarre Adventure: Heritage to the Future.
- Mad Dummy itself has similarities to Dio Brando; it has an aggressive, boisterous demeanor and difficulty caring about anyone but itself (notably forgetting the name of its cousin despite vowing to avenge them, instead of focusing on its desire to escape the Underground and take its place in a storefront window). Mad Dummy also attacks the protagonist with a knife when its usual means of combat fail, and it often repeats words that are similar to the word "useless" (Dio's usual battle cry is muda, the Japanese word for "useless," repeated at a rapid pace.)
- This reference is clearer in the Japanese translation of Undertale, where the script uses "無駄," the exact phrase and letters used by Dio.
- It is stated that one of Napstablook's cousins also went to possess a training dummy, but it is not confirmed if either Mad Dummy or their cousin is related to Napstablook's cousin at all. In fact, after interrupting the battle, Napstablook refers to Mad Dummy as the protagonist's friend, suggesting that the two might not be related.
- Like the Dummy in the Ruins, repeatedly missing attacks/checking Glad Dummy makes them "tired of the protagonist's aimless shenanigans," and they leave.
- It is believed that Mad Dummy possessed a life-size Mew Mew doll sometime after the fight in Waterfall, thus becoming Mad Mew Mew, the Nintendo Switch exclusive boss.