the mob left

They are all black, literally, completely black skinned. In short, in the city there are people with completely black skin and normal skinned people, or not only people.

Mob knows that Twixel can transform from a human into a cat or vice versa, but Grox didn't know about it and wasn't told, so he didn't know anything and won't find out.

Well, the mob is essentially the owner of the Twixel, only if it is in the form of a cat.

He has a black skeleton mask on his head, a black jacket, black pants, dark brown hair, likes to joke, often swears, and is a friend of the mob.

Short black hair, emerald green eyes.
He wears a hoodie with the hood always up. He's always wearing a red-striped bandana that covers half his face, so basically, only his eyes are visible.
He wears red bandages on his hoodie (for appearance's sake), and the left sleeve of his hoodie is slightly frayed.
Dark pants with two red ribbons on the left thigh above the pants.

A normal cat, only black with grey eyes, understands people, and can turn into a human when he wants

The demon guy is 176 cm tall and weighs 35 kg.
His hair is black, short and tousled, with two strands that are longer than his shoulders. He has small horns on his head, a thin tail, and very small wings that Twixel can't use to fly.
Wears a dark blue scarf with white stripes, a dark sweater and dark pants
The tail, although thin, is strong, so it can wrap around an arm or leg, sometimes even around the waist, but very rarely, but it only wraps around a mob, or it can simply detain a person, for example, if he wants to run away while he is not looking.
thin waist.
not very sharp fangs