While I'm doing late night media analysis I do think it's really significant that undertale focuses on children as the protagonists while deltarune focuses on teens. Bc on a meta level undertale feels more like a lesson being taught to the player - positive actions have positive consequences, harmful actions have harmful consequences (there's more nuance than that but on a basic level) - but from an in-universe perspective there's this sense of guidance in the relationship between frisk and the player, once you realise that frisk is in fact a separate entity from yourself. Frisk pretty much just does what you tell them to do no questions asked, because they're just a kid, and the route that you set them on does seem to influence their personality and outlook if you go by the differences in flavour text between routes. The point is, you're guiding and shaping them throughout the game, and there's not really any resistance to that
But then you get to deltarune, and this time you're faced with a teenager who very clearly already has their own life and identity that you're infringing on. Your influence stops being guidance and becomes unwanted control, and that invites rebellion, which seems to be a recurring theme in the chapters so far, and is a really familiar one for teenage narratives - rebellion, identity, autonomy, and specifically the denial of those latter two by authority figures which results in the former, but I think the really interesting thing that deltarune does with regards to kris at least is that it makes you, the player, the authority figure. You're exerting control over their life in a completely literal sense, and even if you're making choices in their best interests, they're still choices that you're making for them with only the smallest amount of wiggle room on their part. To me their acts of rebellion against the player make perfect sense in this context - it's a textbook teenage narrative played out in a very literal (& therefore pretty disturbing) manner and I'm really curious to see what the rest of the chapters do with this setup