His best friend could be a main character. Maybe his friend is like a side - kick in the 'game' of his life, always there to help him with quests or give him advice that's like hints in a game.
The Christmas story in Luke tells of the birth of Jesus, including details like the angel's announcement to Mary and the shepherds visiting the newborn.
Their relationship could develop through mutual support. Since they are having a baby together, they would need to be there for each other. For example, Luke could be there for Jessie during her morning sickness.
One of Luke's main allies could be a new character, let's say a scientist named Dr. Mills. Dr. Mills has been studying the walkers and has some knowledge about their weaknesses. Also, a group of children that Luke stumbles upon could become his allies. They might not seem like much at first, but they have a great sense of the surrounding areas and can help Luke find food and shelter. And of course, there's always the possibility of an animal ally, like a loyal dog that warns Luke of approaching danger.
Luke's friends from the light side could also be important. Characters like Leia, Han Solo, or Chewbacca. Their reactions to Luke's turn and their attempts to bring him back would be significant aspects of the story. There could also be new characters created specifically for this novel, perhaps some Jedi who were thought to be dead but are actually in hiding and oppose Luke's new path.
At first, it could be quite rocky if they find out later in the story. Percy might be angry that Luke has been keeping secrets or has done bad things in the past. But eventually, they would likely grow closer as they realize they are family and start to support each other.
For Katie, she could be developed by giving her more complex emotions. For example, instead of just being a meek character, she could have moments of great strength and determination. Maybe she has a secret dream that she's been too afraid to pursue until she meets Luke. Luke, on the other hand, could be developed by showing his insecurities. Despite his seemingly confident exterior, he could be afraid of not living up to his family's expectations. Their relationship could then be a journey of them helping each other overcome their respective fears.