If you assume the Lightner/Darkner duality from Deltarune is analogous to the Player/NPC duality from Undertale, you will see that Ralsei essentially has a very similar worldview as Flowey: fictional characters are just toys meant to played with until they're broken or until you get bored, and there is nothing more to them. They're beings meant to serve our desires.
Ralsei just expresses that in a way that isn't overtly spooky like how Flowey does, and unlike him Ralsei seems to be aware he is also just a fictional character.