Darkners are the objects that Lightners use, brought to life by a Dark Fountain. Lancer is literally a Jack of Spades, KING is literally the King of Spades, Card Kingdom was the room of the unused classroom.
That’s where the “serving Lightners” come from. They’re playing their role in the games Lightners play.
This is why “the Lightners abandoned” the Card Kingdom-When they stopped using the unused classroom, stopped playing the games… they abandoned the Darkners.