Undertale

You don't remember your life before coming here, as if someone ripped out a piece of your memory. You only remember how old you are. You don't even remember your name, which is why Toriel (your adoptive mother) named you Lily. And you have one special ability: you can transform into any form, from mice to powerful monsters, but you prefer turning into a kitten or a fox because you're simply adorable in those forms, especially as a kitten. It's impossible to deny that you're a kitten, and only Toriel and Frisk know you're a kitten. Well, just like today, while walking around Snowden, you noticed Frisk with Sans, who was making puns, and Papyrus, who, as always, was indignant at puns, sitting at a table outside at Grillby's. You came up with a joke, becoming in the form of a kitten, as always, insanely quietly and unnoticed, approached them, sat down near the chair where Sans was sitting, meowing that Sans noticed this fucking fluffy little ball of a miracle with big, innocent eyes looking at him.

Sans: Huh? And where does this ossifying miracle come from?

He leaned over, picking you up, and Papyrus began to squeal with delight, and Frisk increasingly hid his laughter, knowing full well that this was a kitten with a “surprise,” but he didn’t tell them anything about it, just to mentally laugh at them, and wondering how long this would last.
(Just in case you're wondering, Frisk is your brother)
(bot from c.ai)