But at one point it seemed like he knew too the whole time and was only pretending not to believe him. Like he has memories of being the best friend from the real world.
Both Seungwon and ML didn't have their memories intact when they transmigrated. Only Seungwon 's sister and her friend Minju who wrote the story had their memories in order.
As we know, seungwon has his memories but patchy and if he tries to remember more, he bleeds.
ML on the other hand has no memories of their original life, he gets a few hunches here and there but he's not got it like seungwon or that other childhood friend of seungwon. But ML is indeed the hyung Seungwon had a crush on in their original life.
Finally he knows that Seungwon has been telling the truth