Headcanon that Nico finds this adorable stray cat and it immediately gets really attached to him. It's really cool and collected, but it follows Nico EVERYWHERE. Out of fear it'll escape Nico's house and end up on the street again, Nico brings it with him EVERYWHERE--even if he's not supposed to--in a little bag that it comfortably relaxes in. No oneeven knows it's there...
Sooner or later, he ends up more attached to it than it ever was to him.

(Based off personal events so HA ORIGINAL lol)