Rook

One of the new characters introduced is a character named Seth, who as I can tell is the dark world version of the purple flower in Asgore's flower shop

What I noticed is that Seth is very similar in personality to Klug, a character from the Puyo Puyo series, like even more so than Berdly

Exhibit A: They both have pretty similar sprites. Just look at this... (Note that because chapter 5 is literally less than 24 hours old Seth's sprites are from cropped screenshots of the battle against him and another character named Aqua since none of his sprites have been uploaded to the spriters resource yet)

All three of those sprites are from the same battle, and the first one also shows up outside of battle during dialogue, also all of Klug's sprites are from different games.

Exhibit B: Seth's interactions with Aqua (another character introduced this chapter) are very similar to how Klug interacts with the other primp students, mostly Amitie

Aqua is very carefree compared to Seth, mostly wanting to play, have fun, and make friends. At one point she gets confused because Seth is insisting that the Fun Gang are their enemies, not friends like Aqua had decided they were. Aqua literally asks you if there is a difference to the meanings of Friend and Enemy because Seth described it as Friends are people you enjoy having around, Enemies are people you stab with a knife. Aqua's main means of attacking is with knives, she literally has the patience soul's attack pattern from the Omega Flowey fight in Undertale (Seth, by extension, has the Perseverance soul's attack pattern). I think no matter if you say that there is a difference or not the battle against Aqua and Seth plays out.

At one part in said fight Seth tells Aqua to prepare That attack, and Aqua thinks Seth means an attack that is literally just a duck slowly floating from one end of the board to another. Seth gets annoyed by this wondering why the hell Aqua thought they meant the duck attack instead of whatever attack Seth actually meant.

I could go on and on with more of their interactions with each other (as most of the second area of the chapter is spent with them) but we'll be here all day if I did that.

Now for all the Deltarune fans who found this post that don't know what a Puyo is I will describe just who Klug is for you guys.

Klug is a somewhat obnoxious and bespectacled honors student at Primp Magic School who cares about his ego and public image probably a little too much. He takes great pride in his smarts to the point where he commonly overestimates his own knowledge like in Sig's route in Puyo Puyo 20th anniversary where his is unable to give a definitive answer to why Sig's left arm is red, and will often make an excuse to not say that he doesn't know something, such as in the book "Amitie and the mysterious egg" where he is unable to exactly define what the small little bird Amitie found that she named Tama is aside from that fact that it sure indeed is a bird.

Klug also very often ends up arguing with his fellow classmates, mostly Raffina but he'll sometimes argue with Amitie and Sig despite the fact those two don't really argue back all that much, Raffina's really the only one of his classmates that he will bicker back and forth with. Klug and Raffina don't really get along all that well, especially during the earlier games (they get along a bit better nowadays)