Here is the explanation:
Frisk knows Tori because she was one of the first monsters they met.
Tori knows Sans because they met at the door.
Sans knows Paps because they are brothers.
Paps knows Undyne because they "Practice" together.
Undyne knows Alphys because apparently Alphys almost committed suicide in the dump.
Alphys knows Asgore because she is Asgore's royal scientist.
Asgore knows Asriel because he is Asgore's son.
Asriel knows Frisk because... IDK maybe like their soul or something is chara or something? (I don't know the lore)
Its all been one huge circle!

IT ALL MAKES SENSE

IT ALL MAKES SENSE

LORE

(Meanwhile undertale yellow)
Clover knows Dalv and Martlet.
Dalv knows no one.
Martlet knows Chujin.
Chujin knows Asgore and Ceroba.
Ceroba knows Starlo.
Asgore knows Flowey.
Flowey knows Clover
Confusing as f**k.

its a backwards e

dalv knows chujin and ceroba, he was with kanako (and chujin delivered the corn)

bro respawned

It's the circle of life!

Step 3) yeet the child

Bro got materialized

bro respawned