Monster Cats have both a human form and a cat form as well as feline features. They seem to behave like traditional cats, having an affinity for eating fish.
The soul of a Monster Cat has been seen to be very witch-like in appearance, possessing a purple hue in color in the anime. In the manga, however, it is indistinguishable from that of a human soul in color.
One of the earliest hints that a cat such as Blair is a monster occurs when Liz Thompson explains to Death the Kid that his fear of asymmetry is analogous to her own fear of monsters, as she imagines, among other monsters, Blair and also the Flying Dutchman.