LUKE HAS MADE A CUSTOM LIGHTSABER BEFORE
Lightsabers are the coolest weapons in the Star Wars universe, that goes without saying. We know that they are hard to make (that’s why every nerf herder doesn’t have one) and the only person that we’ve seen make their own… is Luke.
After he had his hand cut off and he lost his father’s lightsaber (spoilers!), Luke built his very own replacement lightsaber. It’s blatantly obvious that Kylo Ren’s lightsaber is not standard issue. So where did it come from?
The strange answer is that Luke made it for Kylo. red lightsabers are only used by Sith lords so we doubt that a good guy would just make a red lightsaber for a random person. The better answer is that Kylo made the lightsaber because he’s made a lightsaber before… because he’s Luke.