Theo

Toby said he created Toriel because he wanted a morher character with an actual presence in the story as opposed to mothers who only exist to say goodbye to their kids as they run off into danger.

There's this thing in stories with child or teen characters where adults are locked outside the "competence zone" basically: they can't fix the problems of the story, because that is up to our young protagonists. This is what leads to the abundance of absent or dead parents in these kinds of stories.

Both Undertale and Deltarune are stories with child protagonists, which means that by making Toriel a character with a real presence in the narrative Toby's created a problem: how do you write a character who would desperately want to solve all the protagonist's problems for them, but by virtue of the story can't be allowed to do so?

In Undertale, it becomes a tragedy. Toriel wants to keep Frisk safe, but can't leave the ruins and can't stop them from doing so. She tries, but you beat her, and she's genuinely convinced she's letting you walk to your death.

In Deltarune, it's a different kind of tragedy. Undertale!Toriel knows exactly what danger Frisk is walking into, Deltarune!Toriel doesn't. Because of this, she actually starts out much more conventional than Undertale!Toriel, dropping Kris off at school before vanishing from chapter 1's story. She calls and worries for Kris, but buys it when they lie to her and backs off. In chapter 2 she isn't worried at all. She is completely kept out of the main narrative.

But Toby created her as an antithesis to this kind of trope, so it doesn't stay that way. She is absent in chapter 3, but now that absence feels strange and artificial. Everyone was expecting her to be a party member, but instead she is kept blissfully asleep and unaware, literally trapped. It's not natural that Toriel is so uninvolved. She wants to be involved. She's been worried about Kris for a long time, has noticed they aren't quite like themselves now that we're controlling them, and is trying to figure out what's wrong. But she can't. She's not allowed to know, because she's not in the competence zone. She can't fix Kris' problems. She can't help them.

I think what people miss about the ending of chapter 4 is that the parent-child dynamic has been out of whack with Kris and Toriel for long before that. Kris has not been able to truly rely on Toriel for a long time because for whatever reason they can't tell her about what is going on with them, Carol, the knight, and us. Toriel is not allowed to know, and therefore she can't be the pillar of support a parent is supposed to be for their child. She wants to help, desperately so, but neither the other characters nor the very fabric of story itself will allow her to.

Worse, it's up to the kids to save her. Children shouldn't ever have to be the one to save adults, but in a story where the Competency Zone is set to teens, they have to. Deltarune can't actually become a story where the adults solve all the kids' problems for them, but neither does it want to be a story where the implicatons of how fucked up this is are shrugged off. So instead it makes the implicit trope explicit and explores its fallouts. A child not being able to rely on the adults around them and instead having to be the one to save them destroys the ability of that child to trust said adults.

The moment we discover her dancing with Sans is engineered to hit as hard as possible. Kris and Susie have spent the entire chapter worried about Toriel, and are at an absolute low point emotionally. And she'd been just fine the whole time! The asymmetry is what makes it hit so hard. The kids were worried sick about her but she wasn't worried about them, even though she was fine while they were very much in danger.

I think this moment exists to wake the audience up to what has already been happening. Kids lying to adults and saving them are both common tropes in stories with young protagonists. The audience is accustomed to them, and it won't register as fucked up. But a mother dancing and getting drunk completely unaware that her child is being put through the torment nexus does. The internet shitstorm that happened after irked me because instead of a narrative reevaluation what i saw was people theorizing about Toriel being some kind of chronic abusive alcoholic and how everything would've been fine if she's just been a perfect mother like they thought she was. It wouldn't have been. Even if she'd been sick with worry and calling Kris constantly, it wouldn't have helped. She's never been able to help.

There's a lot of Watsonian defenses for Toriel having fun with sans instead of worrying. She thought they were still over at the Holidays, or the last time she called Kris when they stayed out late she got blown off and told not to worry and she's overcorrecting in trying to be "chill" instead of babying kris, she's a single parent with a depressed child and Asgore—due to his own problems— is not pulling his weight in taking care of Kris and she can tell they're struggling but she can't help and the stress got so bad that when she finally lets go she completely lets go. But we're not looking at it watsonically. No matter what the cause, the impact of the scene is the same. It's a crystallization of what has been true the whole time: you can't rely on your mom to help you. The story won't allow it.