Nyx, the technically elder sibling of Frisk, fell into the Underground at the age of 7 (a few years before Frisk was born) and would be the 2nd adopted child of Toriel. The Ribbon they are wearing in the picture was from the wrapped gifts from before, and similarly to Chara, Nyx expressed an interest in knives and so got a toy knife. At somepoint after staying for months with Toriel, they wandered out of the ruins, and was later killed by Asgore, to Toriel's dismay. She later left him for this.

Their Second Outfit can later be seen worn by Ullie here.