Frisk was confused. There was a new monster in the Underground, a cat monster who carried an odd magnet like tool that seemed to do just about everything. Except… she denied that she was a "monster" at all, saying that she was a cat person, which was a race who lived out in space.
When she asked Frisk what the planet's name was - apparently she was not from the planet - and was told that the planet's name was Earth, the cat monster - person? - grimaced and replied that that was impossible.
When Frisk asked why, the cat person replied, "Because Earth was destroyed. Has your colony not received word about it?"
Frisk was confused about it, but payed no mind. They went through the True Pacifist route.
When they faced Asriel, the cat person stood alongside them, surprising both the human and their ghost companion as the cat girl did not get absorbed like the rest of the Underground. Maybe she wasn't a monster, just like she said.
Then, after they went to the Surface, Frisk Reset.
When they ran into the cat person again, she was awfully confused.
"Didn't monsters just get freed from here?" was the first question thrown at them.
"You remember?" was Frisk's reply.
"I asked Sans about it, but he just… got all quiet and then ran off. Asking Papyrus got me even more confused, 'cause he said that I must've had an awfully realistic dream, besides the whole goat kid turning into a god like being," was the cat girl's answer. "But then I saw you, and knew that it couldn't be just a dream. Cat folk, even raised by humans, do not get visions like that."
Still, Frisk went through the True Pacifist route again, and the cat girl caught on.
The next Reset, Frisk met the cat girl when Sans found them.
The fourth Reset, she was just outside the Ruin's exit.
The fifth, she was taking a nap next to Toriel's fireplace. Toriel explained that she had found the cat girl the day before, waking up on the bed of golden flowers.
The sixth, Frisk was saved from dying, the cat girl quickly scaring off the monsters.
The seventh, the cat girl was waiting at the end of the long hallway, telling Toriel she would watch Frisk while she ran her errands.
The eighth, the cat girl was on the other side of the spike bridge, examining the puzzle.
Reset after Reset, Frisk found the cat girl getting closer and closer to the bed of golden flowers. Eventually, she was shaking Frisk awake, dodging the question, "Why are you here?" with, "Goat mom asked me to check if a human had fallen."
Like Frisk, the cat girl had taken up calling Toriel "Goat mom." Again, like Frisk, she never said her name, but unlike Frisk, she didn't even tell it when they got to the Surface. She just went by "Kittie."
Eventually, even Flowey caught on that "Kittie" was really odd.
Hundreds of Resets later, Kittie was starting to act like Sans, in an odd way. She always had a fake grin - Frisk knew Kittie better than most did, even if she never told her past. She was slowly becoming more and more distant… until she stopped talking altogether. She became silent, stopped smiling, and didn't seem to know where she was sometimes.
That is, until Frisk achieved the True Pacifist ending once last time, and destroyed the Reset button. Kittie had been with them at the time.
Then, a beam of light fell onto the planet, followed by another. The first revealed a winged raptor, the other, a bird woman. Both exclaimed in surprise and joy, rushing and tackling Kittie in a bear hug. For the first time in many Resets, Kittie smiled, and started crying.
The other two off worlders told her that things had been really odd since Kittie had disappeared, time had been resetting! Kittie replied, "I know… and I think that because of Frisk's sacrifice, time will be back to normal…"
And then, the off worlders nodded to Frisk, and the three of them beamed away, and that was the last that Frisk saw of Kittie… But they knew they would see her again.