Her robe is black and blood red, and is frayed at the bottom. Her eyes are wide open and yellow, with her irises being a bright red color, giving her a look of insanity or exhaustion.
Toriel is seen to be similar as the original Toriel; motherly and caring. However, under the most minimal tension, she may become aggressive. She tries to keep the children for herself, always offering love and care. However, when the child inevitably tries to leave, she will fight them and attempt to kill them to offer them a comparatively quick death, rather than a long and painful death at the hands of Asgore or any other monsters.
Unlike the original Toriel, the Underfell version doesn't know how to cook well and messes up her recipes. She uses horrific substitutions as ingredients, such as blood, monster dust, etc. She asks Frisk a lot of questions in a confused and disarrayed way.
Toriel cares for Frisk while they are in the Ruins, but she tortures Frisk and tries to give them "A painless end" when they try to leave.
Asgore is Toriel's EX- husband. She divorced Asgore because of him killing off each and every human that fell into the underground. Now Toriel gives humans "A painless end" whenever a human tries to leave the Ruins.
Flowey / Asriel is Toriel and Asgore's son. Toriel doesn't like Flowey because of his kindness. In the Underfell Fangame, Toriel helps you in some part of the Ruins if you killed off Flowey.
Chara was the adopted child of Toriel and Asgore. Really not much is known right now but more will be known soon.
Sans and Toriel usually talk through the exit door of the Ruins. They usually talk about how the Underground has gotten worse.