I might just be overreading Flowey as a character. My first interpretation was just "oh boy here we go, gotta get the avatar to tell me I'm a bad person", but maybe that's just me having Kojima fatigue at the time.
While this is pretty late, since I know where you're coming from now I can give my own answer to your original question. Deltarune doesn't really have any character who comes close to breaking the fourth wall like the Flowey or Sans do, but the game's writing does have a meta element to it, but it's different from Undertale. In Deltarune it gets a little meta with how it plays with the idea of freedom that's present through out the plot.
The game starts with an unknown person talking to you, the player, directly to guide you through a character creation process, but after making your "vessel" the voice is interrupted by another voice telling you your vessel has been thrown out because no one can choose who they are, at which point the game shifts to you controlling the game's pre-set protagonist Kris. The rest of chapter 1 just seems normal as far as you controlling Kris like he's a silent JRPG protagonist with some minor dialogue choices, but starting with chapter 1's end scene and into later chapters it becomes clear that the soul inside him (it pops up as a heart in battle or dialogue options like in Undertale) is not his own soul. So far, it's implied the soul was meant to go in the body you created at the start, but was redirected somehow to go into Kris.
You, the player, aren't actually controlling Kris directly, you're controlling the soul, which can influence what Kris does and says. So this leads into a sort of meta conflict, as what you and Kris want isn't necessarily the same, and Kris will sometimes subvert the actions you choose to do, like taking a choice literally in way that doesn't have its intended effect (like you choose to open a door so you can look into a room, but Kris opens it with his eyes closed so the narration can't actually describe what's in the room), or if he really doesn't want to say a dialogue choice he'll do something like muffle it under a fake cough or quietly mumble it with his mouth closed. It also causes some character who know Kris already to occasionally comment he's acting strange, because of the player having him do things he wouldn't normally do.
This ends up tying into the freedom idea. There's a prophecy that ends in some kind of tragedy that several characters want to avoid, which is talked about times ("I want to believe there's not just one ending!") in way that brings to mind a player not wanting to be stuck following the game's plot to a pre-set bad ending. Some characters have a fatalistic view on things, sometimes leaning on the fourth wall with telling you your choices don't matter, and on the other hand there's some characters that do crazy things in the hopes of attaining some kind of sought-after freedom. One major character in chapter 2 talks to Kris about this want for freedom in a way that comes off as layered, as it can refer to Kris wanting to be free from the soul's (your) influence, but can also be taken as referring to how you, the player, want the freedom change how the story goes. It seems to want to give the sense that you, the player, likely want your game choices to matter and allow you to break out of the prophecy along with your party members, but you're constantly frustrated by things happening in ways that negate or preempt your choices so you have no major affect on the plot, ie. the prophecy keeps going, with the antagonists (
including Kris it seems subverting your choices) wanting to keep things on track. And addtionally, the game does have a secret alternate route you can enter in chapter 2 that so far has major changes to that chapter and chapter 4, but it's kind of an evil route that comes with the implication that's it's possibly breaking the prophecy for the worse rather than the better, but it's so far the only way you can achieve the "freedom" to change how things go.
