In the original timeline, the protag was OP and had all the cheats and heaven's favor. As for MC and ML's history before transmigration, see my post here.
The process of making up is slow. ML sacrifices a lot (even tearing off bits of his soul to watch over MC) to keep MC safe. MC is confused because his master doesn't seem to favor the protag like he did in the other timeline. Eventually, MC realizes that ML DOES care, he's just good at denial and extremely (excessively) introverted. MC has to push and push to get through ML's tsundere barrier until it finally breaks and they get some words straight. At one point MC even gets stabbed; good thing ML was worried despite himself and went back to check -- he found out that MC was weaker than he let on so the stab did more damage than it should have. His stone face completely broke and he called for emergency help immediately. The doctor lady mentions that it was the first time she'd ever seen him afraid, not to mention to that degree.