That laugh's an easy coping mechanism, even if it's just a shade of the Real Chara's smiles and too-cheerful looks. Frisk's breath tightens as they reach to write again.

* I know all theirs too.
* Don't worry.
* Just let me know if they hurt you again. Okay?
It's braver than that neutral tone of a marker might suggest. Frisk is still shaking subtly, worried, scared, nervous, knowing that once this event ends, whenever it does, it'll all go back to how it was. Their mirrored self being brutal and sarcastic and thoroughly nasty toward Chara's mirror, and they won't know anything about it.