Idk what's really the whole story but ive read a lot of theory, and what the vibe im getting is that maybe the two of them are really close and that at some point the black haird ml died and or trying to kill himself/someone killed him and the brown hair ml tried to put him back together/ revive him, that or he tried to save him several times, they started to like each other or the black hair ml started to get attached to the brown hair ml and since it looked like the black hair ml is experienced, time travel or reincarnation(restarting life again) must've been involved, that or he just adapted very quickly, that or someting else. anyway, back to reincarnation he might've known that he would meet the brown hair ml on that place hence why he stayed there like waiting for him or something. idk really it might've been the translation but I get the vibe that brown hair ml may have something to do with the scars or that he was really close to that black hair ml in other either past or furutre timeline, but I really doubt they are close to each other before the zombie apocalypse, that or maybe he has amnesia which i highly doubt. that's just my take.