One thing I love about Undertale is the flow of the bosses/cast. Each character has some sort of connection to the other by someway. Toriel is Sans best friend (Sort of) and he talks to her about his brother as well so she has some sort of knowledge/connection to the brothers. Sarns goes along with his brothers antics, Papyrus knows Undyn because she is his mentor and inspiration, He also knows Alphys due to him using some of her machinery in the Snowden puzzle area, in return they both know in. Undyn is Papyruses teacher and is friends with Alphys. Alphys gave Papyrus her puzzle machine, and her love interest is Undyn and that speaks for it self. Alphys is also sort of friends with Mettaton and built his robot body. Finally all of these characters work for KING ASGORE. Whose their boss and also the ex-husband of Toriel.

You could also look at it from an introduction stand point: Toriel knows Sans, Sans knows Papyrus, Papyrus knows Undyn, Undyn knows Alphys, Alphys knows Mettaton, And they all know KING ASGORE. Who is set up to be the final boss. Until it it later revealed to be Flowey/Asriel who has a connection to all of the most of the cast.

Toriel & Asgore are Asriel's parants

Flowey is Papyruses best friend

Sans knows about Flowey to an extent and it's implied to have battled each other in previous timelines

Alphys created Flowey

And the rest are probably characters Flowey has interacted with in pervious runs

This is something I feel Undertale Yellow lacks, Character Flow. Yeah sure Dalv knows Kanako and maybe sort of Ceroba and Chujin but at the same time they don't ever reference him other than Chujins entry logs. I feel like this could be fixed with something like this.

Dalv- Knows Kanako, Ceroba, and Chujin. He could have been used to foreshadow or give mention to the final boss of the pacifist run which is Ceroba liek Toriel did with ASGORE in her neutral death cutscene Marlet could have some sort of connection to the fisty five by maybe hearing reports of some "Trouble makers" in a deeper part of the underground. And add more tension to their proper introduction to each other. Starlo knows Ceroba and her family due to them being best friends and Starlo having a crush on her in the past.

For the min bosses. . . I have no clue lol. Ones a freaking Bat and the other is some sort of DANCE DANCE REVOLUTION weeb.