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Tear Jerker: Sword Art Online
Volume Seven. All of it, both good (Asuna's reconciliation with her mother, Kirito's Big Damn HeroesYou Shall Not Pass, every damn heartwarming moment between Asuna and Yuuki)and heartbreaking.
Special mention has to go out to the end of the novel here, it is in this Troper's humble opinion the single most moving ending to any novel they have ever read Yuuki is finally succumbing to her lifelong battle against AIDS, and Asuna convinces the Doctor to let Yuuki log into the Medicube one last time before the end. They meet at the same spot where the originally met, and Yuuki gives Asuna Mother's Rosario, her one of a kind 11 hit combo. Following this the Sleeping Knights guild show up and pay their last respects, and Kirito & Co. do the same. Finally in one of the most heartwarming and breaking scenes in literature over 1000 players from the game put aside any conflicts and come to say goodbye to the Absolute Sword, who fades away after thanking Asuna for all she did and saying "I tried my best to live... Here, I really lived...". And to top it all off at her Real Life funeral over 200 players arrive in the flesh, surprising everyone at the hospital who thought she had no friends. Excuse me I need a moment...
Red-Nosed Reindeer, one of the sidestories. After Kirito joins the Black Cats of the Full Moon, they all get annihilated, including Sachi, a girl who reminds him of his sister, and someone he swore to protect. He goes through plenty of shit to find an ultra rare item that's supposed to resurrect dead players, and eventually gets it with the help of Cline and Furinkazan.... and learns it only works ten seconds after death. By that point, Sachi has been dead for six months.
Guess what story is the basis of episode 3 of the anime? You will never think of the song mentioned in the title the same way again.
Specially heartbreaking the moment when Sacchi actually hums the song while Kirito utterly breaks into Manly Tears. Bonus points for the darkly sad and lonely atmosphere of that inn room during Christmas Eve. You can argue the story is rushed for trying to fit everything in one single episode, but that specific moment totally knocked it out of the park.
Pina's death is heartbreaking, especially to pet owners.
Lyfa guessing who Kirito really is. Lyfa/Suguha was preparing to forget her love for Kazuto by trying to love Kirito. She entered a brief Heroic BSOD when she finds out Kirito=Kazuto.
Kirito's first attempt to charge the barrier over the World Tree is heartbreaking. Even as he bounces off of it, he just repeatedly throws himself at it, showing just how desperate he is to reach the woman he loves.
Not to mention the sight of Yui pounding her hands on the barrier, frantically calling out for her mommy.
Episode 12. That is all. Yui's reveal and Disney Death. She comes back in ALO but still...
In episode 14, Asuna dies in Kirito's arms after taking a strike meant for him. He picks up the fallen sword, surely ready to start a Roaring Rampage of Revenge...and swings weakly at his opponent. Just...no, strength, no speed, no technique. His opponent barely even has to dodge. He is clearly completely broken.
Episode 22. Kirito is killed by the Guardians...and, when Lefa resurrects him...she realizes Kirito=Kazuto, and breaks. Kirito is also shattered by his failure, and about to just walk back in and try AGAIN.