Toriel (a.k.a. goat mom) is a major character in Undertale. She was previously the ruler of the Underground, but because her husband Asgore declared on humanity and planned to kill any human child to fell to the Underground, she left her "New Home" and headed back to the Ruins.
Born Princess Toriel Dreemurr, she was the daughter of King Victor III and Queen Ariel V. She went through a rebel phase in her teen years and had a potty mouth from the age of six until she was 17. She eventually fell in love with and married Asgore. After the humans sealed the monsters underground in 1997, Asgore and Toriel married and were crowned the rulers of the Underground.
She agreed to adopt the first fallen human Chara upon Asgore's request, but one thing led to another, and both of her children were killed because of Chara's dumb plan. As much as Toriel was stricken with grief, she couldn't bare to hear Asgore's speech in which he declared war on humanity for this incident. Toriel quietly left her throne and headed back for the ruins.
She would always try to keep the children who fell to the Underground in the Ruins for the rest of their lives, but they all left, only to later be killed by a guard or Asgore himself. This makes her really bad at her job. She did get her boyfriend Sans to promise not to kill the next fallen human, which was probably a good thing in hindsight (though the Genocide Route definitely undermines this).
Once Frisk fell to the Underground, she felt she had to destroy the barrier once and for all. This originally resulted in her dying, but Frisk later reset and did a True Pacifist Run. In the end of the True Pacifist Run, in which the monsters make it back to the surface, Frisk agrees to be adopted by Toriel (because she absolutely hated the human orphanage before falling down). She became a teacher shortly thereafter.
As royalty and a boss monster, she has one of the highest power levels in the Underground at 55. If she would've actually trained, she could've easily surpassed a power level of 100. Sadly, she didn't train in her teenage years, and once she reached adulthood, she grew to simply hate senseless violence. (She'd absolutely hate Itchy & Scratchy.)
She also has the power of pyrokinesis, but she mostly uses this for cooking.
- She and Asgore once won some contest for rubbing their noses together.
- Unlike certain royal families, she is not related to Asgore.