The mad dummy is a ghost that lives in a dummy. In UNDERTALE, UNDERFELL, and OUTERTALE, he can be found in the dump from chapter 3. Regardless of what you do, because you met his cousin that also lives in a dummy from the first chapter, he will find an excuse to be mad at you and try to attack you. Only for you to be saved by Napstablook. (How that transpires in the Fell and Outer AUs is yet to be fully determined, I'm asking fella to give me their "canon" interpretation of the fell mad dummy, so I'll get a response soon, and I haven't quite figured out how the Outer mad dummy will act, but all versions of the dummy will have the same voice) All 3 versions later become mad mew mew, but that'll be a separate audition. In UNDERSWAP, he trades places with his cousin, so his cousin is the one that gets offended in chapter 1, and tries to attack you, only for Asgore to intervene and demand that the dummy apologizes to you. The dummy complies, but still doesn't like you. So he teams up with his cousin in chapter 3 (not that his cousin is very helpful) only until Hapstablook interferes. The swap mad dummy will become mad muscle, but like mad mew mew, that's a separate audition. (I warn you, these characters will do a LOT of yelling, you need to be able to do that for a long time, and also to do it well.)