Undertale is a very efficient, coherent game. It is tightly constructed, in a certain sense, with all its elements being designed in a very particular way to fit the particular experience the game is trying to create. Undertale is dripping with metatext; everywhere there are double meanings, everything connecting to everything else in some interesting way, be it some deep metacommentary or just worldbuilding flavor. Not literally everything... but an amazing number of things.
Toriel is one of the first characters we meet, saving us from Flowey. She is called "Toriel" because she is Undertale's tutorial character- or rather, she is set up for the tutorial character role, which the game then toys with and subverts. She is overbearing, she solves puzzles for us, insists we stay out of danger, acting as a protective motherly figure. The game uses this first for comedy, but simultaneously to build an emotional attachment with the player, culminating on reaching her home with its warm palette and music (and pie). This is all setting up for our battle with her, Undertale's first major boss encounter. We likely feel attached to Toriel, but are forced into a combat scenario where we may have to kill her. Toriel's love is outwardly comforting, but it's also clearly repressive, and the old children's belongings in her house may have raised some red flags. So, we fight her. We may even kill her- the game sets us up to easily do this by mistake. Her death is tragic, tragic in exactly the way Undertale needs it to be, firmly establishing the emotional weight of killing monsters which will haunt us for the rest of the game. It likely sparks us to load our save and try again, setting up for Flowey to call us out, establishing the game's subversion of ordinary RPG systems as narratively relevant tools which are also used in support of the game's themes, establishing the potential for our actions to have more profound consequences in a way most games can't...
If you've played the game, this is probably all obvious to you. I'm just straightforwardly describing the experience. The point is to spell out how tightly woven it is, how precisely Toriel is crafted to fit the particular set of overlapping thematic roles that Undertale needs her to fill. The game is very efficient in this way- everything is serving multiple purposes, having layers of meaning, even (or especially) things which we are predisposed not to expect narrative meaning from- the save system, the battle UI, the narrator...
This is where Deltarune immediately loses me.
Torial appears as a major character in Deltarune as well. She has a role in the story, she isn't pointless. But her inclusion feels a bit... hollow, to me? She is no longer a character finely crafted to fit the particular experience of this game; rather, she's just here because all the Undertale characters are here. The game makes use of her, but it doesn't have nearly the same elegance that made Undertale so special.
Another more basic example: In Undertale, the characters are mostly monsters. In-universe, this is because the player character has fallen into the world of monsters, which has a particular narrative significance with how monsters are different from humans as a species, and a historical relationship between species which sets up the game's story (the Underground, the barrier). On a meta level, the characters are monsters because the game is playing on expectations from the traditional story of a human being trapped in a dangerous monster world, which are then subverted with the monsters being relatively friendly, once again playing into the game's themes. In Deltarune, on the other hand, the characters are monsters... just because?
So much of Deltarune is like this, and it's a large part of why I initially had a fairly negative view of the game. It wants to establish its own story and identity, but it's still haunted by all of Undertale's iconic characters and imagery, refusing to let it go. Struggling to utilize story elements which were precisely crafted for a particular scenario is something many sequels have difficulty with, I think. LocalScriptMan (er, Hartwell) spelled this out in their video on Stranger Things a few years ago. It's a bit more peculiar in Deltarune's case, though, since it isn't exactly a sequel, but a weird sort of alternate universe story...
Anyway. I like Deltarune. Chapter 1 disappointed me, but 2 totally won me over with its original characters and the beginnings of more interesting narrative and meta concepts, and subsequent chapters have remained pretty consistently appealing. Do I think it'd have been better with all original characters? Maybe. But I understand there are complicated reasons Toby Fox made the choices he did. (Thinking of Sans, for example.) I just wanted to articulate this particular way in which the game feels fundamentally messy to me, and why it's fighting an uphill battle for my investment.