So I Was speaking to another Deltarune fan I Know and they told me a very interesting theory and I Thought I Would share it with you. So in the dark world all the characters sprites change and there dark world sprites are a representation of who they want to be, So if we apply that logic to Kris that mean they want to be a knight (Witch is why they have knight armour) So Kris grows up and speaks to Gaster and Kris says "I Want to be a knight" And then Gaster transforms them into the roaring knight, Then Kris The Roaring Knight uses the busted time machine owned by sans to travel into the past, And that's why Kris and The Roaring Knight are best friends