
I bought the raws on lezhin a while back, will give a peek into the more important chapters:
- Chap 31 it shows the short dark haired guy, at school, being kidnapped by the bullies. He is thrown into a room, masking tape is put over his mouth and he is blindfolded. Then in pops the blond friend, who has been missing from school to save the day. Blond friend removes his wig, and it turns out he has been diagnosed with a brain tumour?
- Chap 32, the blond friend talks about his childhood and gives the short dark guy a mobile phone (as he wont be at school so it's a means to keep in touch likely). Then the friend leaves, and the dark guy is discovered by a group of girls who help him. Then it moves forward to the current day and back to the phone.
- Chap 41. New boyfriend has followed dark haired guy, as he left a note saying he was offski somewhere. They end up outside a big house, and it turns out this is the home of the blond friend from school. Inside the blond friend is on a hospital style bed, and he talks about his upcoming death and makes some jokes. He then points over to a piece of art, which turns out to be the portrait the short dark guy did of him when they were kids.
- Chap 42, funeral of friend and dark haired guy throws the phone in a river. He's made peace with the past basically. It shows him attending an art exhibition of the new boyfriend.

I think so, as I read the raws quite a while ago so had to revisit them. Tried to keep my spoilers very brief as most of the chapters haven't yet been uploaded here. Here is the link to Lezhin Korea https://www.lezhin.com/ko/comic/nobody_knows
The ending is quite positive, for the "main couple" anyway, but yes his childhood friend does die and it appears he lived with his illness for most of his life. There are more details to the plot, but thought I'd just keep to the bigger points.
Please don't tell me that they're going to molest him or something 'cause I'm.kinda getting sick of every uke getting manhandled at some point (╯°Д °)╯╧╧