I read it in japanese since there's no eng ver unfortunately. I'll basically spoil whats missing up until this current chapter since there's a lot that happens during the relationship before the actual break up (even that scene is cut short and downplayed in the manga)
I've read the novel and tbh it makes me sad how the manga cut out SO MUCH important content. There was a whole chapter of them at age 22 from Kai's pov and it shows us how the downfall started and he is a lot worse in the novel, even I have a hard time trying to justify him