Monster Kid is usually depicted with a sharp protrusion on the front of their head and along their tail. Like most other monsters, they have a red and black color scheme for their rags. They have sharp teeth and a black and red sweater.
Monster Kid has always been known as a bully who hates almost everyone. They hate Undyne, Sans, Grillby, Papyrus, their family and even the entire town of Snowdin, which is why they ran away. They try to kill Frisk and Flowey. However, before Monster Kid fights them, and after they were rescued for almost falling down the bridge, Monster Kid saves them, then almost kills Flowey.
In Undertale, they idolize her; but in Underfell they completely hate her and her guts.
They hate the skeleton brothers a lot since they always argue endlessly.
Monster Kid hates and bullies Frisk.
Monster Kid hates Muffet, because she gave them poisoned cakes.