Toriel is a goat-like monster who wears metal armour with the Delta Rune engraved on it. She has long floppy ears, a pair of visible fangs, two small horns, and a purple cape. In her battle, she wields a long, bright-red bident. She wears a tiny crown on top of her head. She resembles a white furred nubian goat.
Toriel is depicted as being a calm queen, and known to be very kind. She feels deep regret for killing children that she wished she could have raised as her own. She will make an offer of pie before the battle, and follow through if the protagonist agrees. As Queen of the Underground, she is often depicted as being overtly motherly to her subjects, old and young alike, tucking them in bed, reading stories to children, and disiplining any troublesome teenagers.
Asgore and Toriel were the king and queen of the Underground before the events of Underswap. They eventually adopted a child, who they named Monster Kid (as great as the queen was, she was never very good at names). They also adopted the first human to fall into the barrier. Once both of her children died, she assumed that all humans were evil, and decided to kill any other human who falls past the barrier.(like regular UT) Asgore hated her decision, and left the throne to protect any humans who fall down in the Ruins.
Toriel is first encountered in the Capital, in her castle. At the beginning of the battle, she uses her bident to destroy the MERCY button, forcing the player to fight her. If the protagonist uses the Peppermint Camomile Tea, the flavour text will say that she remembers something. Once her health is lowered, Chara has the choice to either FIGHT or SPARE her. Either way, she will die. If she is spared, Temmie will appear and kill Toriel. Tem then uses the human SOULs to undergo her transformation into Photoshop Tem.
True Pacifist Route
During the battle, Toriel is interuppted by Asgore who decides that killing Toriel to escape was not worth it. After him, everyone else will come in and then Temmie will come and take the human souls and all monster souls, turning into Monster Kid. Toriel appears as one of the Lost Souls in battle.
Asgore is the former king of the Underground, and Toriel's ex-husband. Like in the original UT they were very close and loved each other very much, as they were always seen being romantic in public. They were a couple until Toriel decided to kill any humans that fell into the Underground. After that, Asgore gave up on being king, broke up with Toriel, and went back to the Ruins.
When Alphys was younger, she challenged Toriel to a fight, which Toriel obviously won. They battled several times, until Toriel offered to train her. She continued to train Alphys until she became the head of the Royal Guard. Now Alphys trains monsters.
Frisk (or whatever the player chooses to name them) is the second adopted child of Toriel, and Monster Kid's best friend.
Undyne is the royal scientist that Toriel had offered her to experiment with Determination and "Create a soul" when really, she just made robotic body for a ghost.