The third saddest moment in the series comes from the Soul Society arc when Ichigo defeats Renji in their rematch. Ichigo proves himself, but it breaks Renji in the process.
Ichigo and his group had broken into Soul Society in order to save Rukia. Ichigo gets separated from the group and finds his way through the Seireitei only to find his way into a wide open area that is being guarded Renji and their rematch begins (we all knew it would come).
Renji states he won’t allow Ichigo to complete his task and the huge battle goes off with the two sides bashing each other with their swords. Ichigo in the end is able to defeat Renji, leaving him on death’s doorstep (he doesn’t die, relax). It’s at this moment as the two warriors begin to collapse that Renji reflects on his true feelings and its where the sad moment begins because we finally learn why Renji takes this entire thing so personally.
Renji feels a deep sadness. He was not able to save Rukia himself after she was punished and sentenced to death, a childhood friend that he had sworn to always protect. He is also not able to defeat Byakuya and prove to not only him, but to himself and Rukia that he was just as powerful and capable. Renji always wanted to save her and now he is defeated and close to death. Realizing that he is done for, he demands Ichigo save Rukia in his place, telling him he must not let her die no matter what. Renji had always blamed Ichigo for Rukia’s punishment, but he realizes that its not his fault and he is doing all he can to save her. Renji swallows his pride and cries out in agony in both his physical and his emotional pain take over.
You see a tough warrior breaking down, realizing his fate, and begging a former enemy to complete a task he failed to accomplish. That is pretty emotional scene, especially when it comes to one of the toughest characters in the entire series.