Toriel Hoperiss-Dreemurr

Early Life
Toriel was raised on a farm by her father, belonging to a long line of royalty that she and her father rejected, though not her by choice. She became infatuated with a human during her early teen years before she met Asgore and learned the dark truth to the human she loved, who turned out to be Lucifer Marchoz. He was fought off by Gaster, who Toriel had become close friends with. Sometime after, she married Asgore around the time when Asgore's father, Asgard Dreemurr, was assassinated, sparking the events of the Great War between humans and monsters. She helmed the war alongside her husband before they lost, and were forced to flee underground or else face extinction.

Underground
Toriel and Asgore eventually decided to trade their immortality for a child, and Asriel was born. Sometime later, they adopted a human who fell underground, named Chara, who became Asriel's sister. During their youth, the children were killed through unknown circumstances, and a legend was born of it. Asgore in his rage, blamed humanity and asserted his dominance that they would wipe humanity out. Disgusted at his aggression after their experience with Chara, Toriel left him and sealed herself away in the Ruins, where she remained until the evacuation of the underground. She witness all six children to fall after Chara, but didn't interact with all of them. As each one was killed by Asgore, Toriel began to grow desperate. When Frisk fell underground and proved her worth and left the Ruins, Toriel felt overcome with guilt, knowing Frisk's SOUL would be the last one Asgore needed to destroy humanity. In a million different timelines she made a million different choices, but the current timeline played out differently. She left the Ruins to find Frisk had elected to remain underground and live among the monsters. Even Asgore allowed it, unable to carry out his duties, Frisk offering a different kind of hope to the underground. However, the CORE destabilized and the underground began to fill with magma, forcing an evacuation. The barrier was broken by the combined efforts of Frisk and Flowey with the six human SOULs, and the monsters were freed in time to escape the eruption of Mt. Ebott.

The Surface
Once on the surface, Toriel offered Frisk to stay with her, the child agreeing. Toriel divorced Asgore and adopted Frisk, though Frisk took on the name of 'Dreemurr' opposed to 'Hoperiss', which irked Toriel, who allowed Asgore to be a part of Frisk's life begrudgingly, holding unnecessary hate in her heart for her ex-husband. She began a teacher at 'Dreamhope Elementary', before she began to uncover secrets. It's unknown how she discovered the Messiah, but when she began to dig them up, the cult put their focus on her.

Episode 1 "A Happy Ending"
Toriel first appears in the story cooking Frisk a birthday breakfast before allowing Sans to take Frisk to school. This is the only time Toriel is seen this episode, but through Frisk it is learned that Toriel has been leading a double life, though only Toriel knows of its purpose as she attempts to uncover and expose the Messiah, understanding the threat they pose and intending on dealing with them before it's too late. Toriel attempted to give Frisk all of the information she had, but her apparent lack of interest in her adopted daughter hurt their relationship enough that it detrimented Toriel's plans, and most of her information gathered was recovered by the Messiah save for a single flash drive that Asgore and Undyne managed to find. Toriel was killed by Lucifer Marchoz elsewhere, who was going by the moniker 'Demon'.