When the Glitch broke over the Underground, those closest to it suffered the most. The Resort and all within it erupted into chaos, overtaking and nearly destroying Mettaton’s EX form in their madness. Catty did not survive the first wave of the Hunger, falling to a pack of starving monsters while attempting to forage through the garbage for food for herself and Bratty.
Bratty lost her mind when she found her friend being torn to pieces by the ravenous pack, and killed them all with the gun she had in their inventory with nigh military precision, saving most of Catty's nearly dead body. She sat with her comatose, vegetative friend in their alcove for months, rocking and talking to her and laughing hysterically, attempting to feed her any food she found or harvested from passing monsters, but to no avail.
Eventually, she ceased to recall anything but Catty, anything even about herself. And in the end, when her mind finally broke entirely, she skinned the cat monster, donned her fur, and, after consuming her entirely, ventured to New Home under her guise. On her journey, she adopted not just Catty's tendencies and voice, but her name as well.
She currently holds a section of the Capitol in check, manipulating and bossing around lesser monsters with sheer charisma. She wields the gun she used to save Catty and a pair of machetes. When she can’t find ammo for the firearm, she slashes into her prey/enemies with said machetes, grinning through razor-sharp fangs and Catty’s empty eyes at her target. She lost a large chunk of her tail at some point, likely chopping it off to eat herself, and instead has sewn Catty's onto the end of hers.
Despite her madness, she is incredibly clever and conniving. She can talk even the most insane monster into obeying her will.