Red is a child-monster (shes a poodle), adopted into the Royal Guard. She wants to kill the player, to implement a plan Asgore had created to destroy the Barrier. Hearing of the Guard Dogs and the player, and their friendly relations, she feared that the player would try to make friends with her and she will do everything in her power to stop them.
Edgy kid Red wears a grey cloak instead so she doesn't get spotted easily but her delta rune is red (like toriels) she still has her bow-tie and looks harmless but all of a sudden she chops you in half with her kantana. she has a red glowing eye like sans which gets red each time thats is a energy bar when it goes white she has the asgore blue orange thing but way more faster.
She has two sides her first side is a sweet harmless girl which gives out (poisoned) muffins. when they know shes to nice is to late there already dust she acts quick and fast as sans dodges. she always obeys Undynes and asgore commands
Despite a supposed lack of hatred towards humankind, her initial interactions with the Human are quite cold. Confident that the Human will be slain at the hands of Asgore (if not earlier), Red shows extreme hesitance in befriending them on any level, even initially refusing to speak, as to "not lose someone else." in genocide route you find out that she sort of likes humans but the rules kill or be killed means she has to kill them (that's her secret know one else knows)
She doesn't member well but her parents were cold bloodedly killed then she grabbed a knife and killed the human. After the war, W.D.Gaster took care for her and gave her power his goal was to make a a monster made for killing but it failed and she was left cold in waterfall stealing from people then undyne trained her since she sure some fighting spirit in Red.