Question?

You mean PaintedFish's mask?

yes

SCP-035, I believe. Y'know what, just read PF's profile on Demirammon, it explains most of it.
https://www.demirramon.com/@PaintedFish

SO
Mask is based off of SCP-035. When the Fish himself fell into the Underground, something something Gaster, Kindness and Perseverance souls, boom, Mask exists now. Mask seeps corrosive fluid and is impervious to physical attack. Mask is in a relationship with Undyne (still a better live story than Twilight) and once did a genocides because Gaster said his soul was weakening.

Recently, through a few Demi's threads, Mask has had the gender switching rock used on him and now Masque exists.

*Applause*
Yep. Correct.

I am very involved in the lore of fictional characters.

good