These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Moral Event Horizon: Gregory for torturing and rendering Neko Zombie as he is now, while keeping him imprisoned in the hellish hotel. And Gregory Mama, for capturing and consuming lost souls all for revitalizing her "beautiful" self.
TV Fish is generally the most benign resident of Gregory House in that it doesn't outright try to kill or harm anyone. But it also projects memories of those it comes in contact with, particularly painful memories...
Neko Zombie may not be especially scary at first glance, but what was done to him is pretty horrific: someone (heavily implied to be Gregory) sewed his mouth, ears, and eyes shut and chained him up in a room in Gregory House, leaving him half-starved most of the time. It's even worse seeing as he's one of the few truly benevolent residents of the hotel.
One-Scene Wonder: A good handful of the characters in the series only pop up in one short for the entire run.
The Second Guest is a bit of this as well. She had just left from a wedding when she found Gregory House, a wedding between her best friend and her ex-boyfriend. Her ex had broken up with her after she said she wanted to work on her career first before she got married, and apparently she never got over it. You can see her slowly beginning to break down because of this as the series goes on.
Haniwa Salaryman and Sleepy Sheep as well.
Even Catherine thinks this of Haniwa in The Bloody Karte.