Jack, TourqueDawg, and Soren all look alike, and Milo looks like the blond version of Soren. And Milo, Lukas, and Nell all look alike.

Are they all siblings of sorts?

Theory: A blonde girl and a orangish-brownish guy got married and had these kids. Then they realized they didn't have the patients of having kids, so Milo went to Sky City with Soren, (And Soren fell off the edge into water, and Milo thought he died. Soren was there until he was a teenager.) Nell went to her world, (Later was taken by Mevia or Hadrian.) TorqueDawg went to the world with two moons, and Lukas went to the overworld, and Jack went wherever he is.

That's my main theory now :D