ok prepare yourself, gosh i just realised how much it hurts me to talk about this ALRIGHT! SPOILERS!!! btw i may be wrong (its been 2 months since i read the novel)
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first of all, take a deep breath in.
There are 3 timelines. If I'm correct, in the second timeline, J's body grows weak like SUPER weak because of the mutation, like so weak to the point where he's like a skeleton. In the end, in the end, he chose to sacrifice himself and throw himself towards the monsters, where he unfortunately dies and leaves sayoung alone (this is actually shown in the manhwa, it's the fragment that yoon ga-eul saw which she then reached out to cha eui-jae.) Anyways, right now we're in the 3rd timeline! I haven't completed the novel, but I'm at ch 320 something and J's hair is getting whiter which means his mutation process may be getting stronger...we don't know how much longer he has left but hopefully sayoung can do something to prevent J from mutating (bc sayoung has his memories from the second timeline despite being in the third timeline)
Also, the timelines happened because in the first timeline, J used a time-reversing watch to rewind time after he lost sayoung (gosh this suicidal couple </3), then I believe sayoung used it in the second timeline but only half of the watch's power worked which is why the second timeline's world is colliding with the third and current timeline's world.
lmk if smth doesn't make sense...sorry for dropping this bomb
I HIGHLY RECOMMEND READING THE NOVEL!!!~ :D

okok so I saw a lot of comments making theories and guesses on our lovely mc's white hair, hate to break it to yall, but it's not bc J spent time in the rift, it's a lot more depressing...read at your own risk... tldr at the end~ (sorry i yap a lot when it comes to thgll)
As we all know, J spent a LONG time in the West Sea Rift, although it was years in the normal earth world, I believe it was like months in the rift, my point being, J didn't spend 8 years in the rift, but time ran differently in the rift compared to the outside world. ANYWAYS back on topic, J was not alone in the rift, he had his team, his comrades - their mission was to enter the rift and 'save' their previous fallen comrades - who entered the rift before them, yet never returned (although J knew that instead of them being medical aid for the pervious comrades, his team was actually there to collect the bodies of his previous comrades since everyone believed they died)
Eventually, they figured out that this rift was extraordinary - not like any other rift. They panicked a bit, but realised they had J (fucking pussys putting their trust in a 20-year-old, shame on you!). Blah blah blah, they couldn't find any bodies, they were getting angry at J?????? (fucking bitches), THE MOST IMPORTANT PART: their food supply was running low. When they ran out of food, they had two choices: eat each other or eat monsters. They chose to eat monsters. So J killed monsters for his comrades, and those bitches ate. BUT THEN ALSO IMPORTANT: one of his comrades turned into a monster (like the ones they killed to eat), that's when J discovered that eating the monsters that they killed starts a mutation process. The monster's blood is WHITE or CLEAR. The more you eat the monster's flesh/blood, the faster the mutation process speeds up. J wanted to tell his comrades to stop eating, but how could he tell them when they were all surviving? He tried to stop them by saying, "Don't you feel weird after eating the monsters?" But they all glared at him, ignored him, and continued eating. J stopped eating the monsters and literally STARVED, but he already had the monster's flesh...the monsters are mutated monsters, and when he ate the mutated monsters, the mutation process began TT~TT
TLDR: the reason why his hair is white is that he ate mutated monsters in the west sea rift without knowing that eating mutated monsters would start HIS mutation process, white hair being the first sign of mutation... TT~TT