In UNDERTALE, Mad Mew Mew is a life-sized doll of Dr. Alphys' favorite anime character, mew mew from mew mew kissie cutie. The doll was in Alphys' lab when the Mad Dummy, a ghost that lives in a dummy, found the doll. The dummy then decided to leave the old dummy behind and inhabit the Mew Mew doll instead, becoming Mad Mew Mew. She (those will be her pronouns as she inhabited a female character doll) In papyrus' tall sink, there is a shrine built by the annoying dog that may be crushed by a large set of doors (in the switch edition and fanmade pc ports), the door can only be unlocked after beating Mettaton from Chapter 4. Upon opening the door, you will find Mad Mew Mew, trying to fuse with the doll body. While the OUTERTALE and UNDERFELL versions of the Character has not been set in stone, the characters will use the same voice as their classic counterpart. There's also a separate role for the Mad Dummy, but I felt like the Doll he later inhabits would make his voice female as it would come out of the voice box built into the doll. It just makes sense in my head, which is why the roles are separate.