Monster Kid is the adoptive child of Asgore and Toriel also being the adoptive sibling of Frisk And Asriel in Underswap. However Asriel was just a baby when Frisk and Monster Kid died who eventually ran away to Sunnedout. They are the final boss in the True Pacifist Route.
Monster Kid is a yellow, reptilian monster that has no arms, two spikes on their head, and a tail. They wear a striped shirt. They also have two marks on their eyes, one being larger than the other one.
God of Hyperdeath
In this form, Monster Kid goes under some changes. Their eyes become black, they have marks under their eyes, and they seem to have grown an extra spike on their head. They wear a long, dark robe with the Delta Rune on the front, with a large collar, and shoulder pads. Their robe looks very similar to the one Asgore wears in Underswap. They also wear a necklace that is similar to the Heart Locket.
Before their death, Monster Kid was a docile, adventurous, and loving child. They cared for Frisk as a sibling, and trusted them. After they had died and became a Temmie, they didn't have a SOUL, meaning they were incapable of feeling love. They use words such as "yo".