The triplets have a friend that comes over for a sleepover. It's a normal affair where they play video games until it's too late to stay awake.
That why when Fenris notices the stranger his sleepy first thought were "this must be a prank." He blinks away annoyance and notices that despite the stranger in their bedroom, the weird humming and the still atmosphere.
Fenris holds down into the annoyance, because the calm does not shutdown the part of his mind that's too critical to take this in stride.
"Do you like my mask? I made it myself," says the figure conversationally.