Quietly Growing Stronger in the World of Titans

Alan, little bro, stop turning evil and just stay with Mikasa.

Given another chance, Rin transmigrates to the world of Attack on Titan. He does have a System, but it's so devious that it won't even give him any cheat abilities. Rin can only rely on himself. Can he change the ending this time and save the lives of the characters he cares about? (According to this novel's rules, Sasha can't die.) Also, the plot won't be changed too drastically, and the protagonist doesn't start out overpowered. This isn't a mindless power fantasy, so enjoy!