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Tearjerker: Digimon Tamers
Because the first two series weren't depressing enough...
Heavy spoilers below; look at your own risk.
Guilmon's first evolution to Growlmon - it was everything Takato imagined, Gone Horribly Right. For a short moment he becomes scared that Guilmon's now a monster and doesn't remember him, and then a little voice pipes up.
"Takato? Why are you crying, Takato?"
Impmon trying to hug Ai and Mako when The Dog Deva, Caturamon, tricks him into thinking he was back at their house. And then watching his heart sink as he sees Ai and Mako fighting over a new puppy just like they did with him...
Leomon's attempt to stop Beelzemon, who saw that Beelzemon wasn't truly evil, but misguided. "To have power is not to be strong", then looking at Jeri reassuring her what their bond means to him. He is then impaled by Beelzemon, and Leomon can only ask why Beelzemon can't see any other way.
Jeri pleading Gallantmon not to kill Beelzemon in revenge:
"B-but if you kill him, we will not be able to revive Leomon, why can't you see that?! I... I just don't want anyone else to die! It's too sad... Please...!"
The English dub is, if anything, even more painful because we start to see Jeri's cheerful facade crumble and her inner loneliness and lack of self-worth break through...
Beelzemon: Why are you protecting me? I killed your partner
Jeri: I hate you for doing that! But even if you died it wouldn't bring Leomon back. It's my fault he's gone, and if you destroyed each other that would be my fault too. I just can't let anyone else... get hurt... because of me...
After reverting from their first Ultimate evolution, Takato hugging Guilmon and telling him that they'll always be together, no matter what. If you know what happens in the end...
All of Jeri's descent into madness and possession is insanely powerful even in the dubs, equal parts terrifying and lip wibblies. Parent death? Check. Parental Abandonment? Check. Revelation that she's been screaming on the inside for a long time and slowly getting worse for the entire series? Check-a-roony! Poor little girl!
When Takato confesses his feelings to Jeri, not realizing yet that it's not really Jeri he's talking to.
What makes this even more heartwrenching is Takato revealing that he feels responsible for everything that's happened, including Leomon's death and Jeri's descent into depression, and apologizing to "Jeri" and practically begging her to just talk to him.
Beelzemon's entire character once he becomes The Atoner. This change leads to one of the season's greatest Crowning Moments of Awesome, followed immediately by one of its biggest Tear Jerkers when Beelzemon uses everything he has to smash through the D-Reaper's defenses, only to be stopped just before he can reach Jeri. Severely injured, his wings shredded, his weapon lost, all out of options, he manages to summon up the attack of her Digimon Leomon's Fist of the Beast King, in a My Name Is Inigo Montoya moment. Not only does he punch through the D-reaper's defenses to Jeri, but hearing him say the attack, and hearing the attack itself, snaps Jeri out of her Heroic BSOD. Just long enough for Beelzemon to yell at her to give him her hand. Jeri looks up, expecting Leomon. Sees the digimon that killed him instead. She's so traumatised that she can't move, and the D-Reaper closes the wound. Beelzemon gets impaled from behind by flying polygons of death. Fatally wounded, Beelzemon plummets to the ground, flickering in and out of existence. And what he says during all of this?
Beelzemon: "Jeri! Take my hand! Take it! Jeri! Jeri! Just let me save you! I need to save you! Jeri! I just need one more chance! Just give me one more chance!"
Jeri: You're... you're not Leomon...
In the japanese dub the scene is, for the most part silent besides the character's as the music score stops playing. This is powerful and effective, but no where near as emotionally manipulating as the score that plays during this scene in the english dub and it cements this scene as one of the English Dubs finest moments.
In the manga version of that scene, she actually does accept Beelzebumon's attempt to help and approaches, but he is cut down before she can take his hand.
How he screams "I need to save you!" on the verge of tears... even though he called his own past actions "unforgivable" he still puts everything he has into trying to rescue her at the repeated risk of his own life is what being The Atoner is about. Thankfully, Jeri does accept him in the end, and the reconciliation between the two is a tearjerking moment in itself.
When Ai and Makoto, having seen the whole thing on TV, take shelter with him. Those two little kids—who are even younger than Suzie, try to reassure him he did great, fighting out there, while he insists he's failed and is going to die. Then they tell him not to....
"You're not Leomon..." You can just feel Beelzemon's heart shatter when Jeri says it.
You can actually see it in his eye's.
It's Played for Laughs at first, but the part where Impmon is trying to get someone, anyone to tell him what the note Ai and Mako left him says. And then when Jiŕnliáng's martial arts instructor reads it out to him. In light of what's happened and what will happen, you can't help but feel your heart swelling.
Impmon: How come you're not scared of me?
Chou-sensei: The body is nothing more than a container. At the very least, I didn't feel an evil heart within you.
Really, the final episode of Tamers from about 4 minutes to about 4 seconds before the ending credits was a Tear Jerker. Watching the Tamer's partners de-digivolve and then disappear into the Digital World, Henry's father's guilt-ridden reaction as he reveals why they're gong away, yet Jianglian still manages to smile at him and then Mr. Lee weeps, and Takato thinking that he'll never be able to keep his promise to Guilmon.
Sadly, if the actual canon is to be believed, the opening that Takato found turned out to be too small for them to actual reunite with their Digimon partners, there is a Firewall keeping humans out of the Digital World that cannot be cracked (without possibly destroying the world), and as of a year after the series ended, the Digimon and their Tamers have not reunited, the closest they get as of this point (the last canon release in this series) was being able to send messages to their Digimon. Talk about a downer.
Which may not even be official canon. Even one of the show's creators has since stated that they like (perhaps even prefer) the "Runaway Locomon" movie and its implication of a reunion, the audio drama has not been expanded upon... Even then, if you consider that it is canon, this doesn't totally mean a solution won't ever be found. But more importantly, it's much easier to end with the Hope Spot that is the show's final moments, because it seems to be closer to what the writers intended rather than the audio drama (so still quite sad, but not soul-crushingly depressing at least).
To make it worse, compare the endings of Adventure & Adventure 02 - In the original series, the DigiDestined were separated from their Digimon, but they at least got to give heartfelt goodbyes to one another & the series ultimately ends on Tai stating his belief that they'll see each other again; & 02 paid that off by having the characters Earn Your Happy Ending with the DigiDestined old & new keeping their partners. Tamers? The Digimon are literally ripped from their partner's arms, after being forced to de-Digivolve, and sent back to the Digital World.
Digimon Tamers's movie had one during the final battle. Shisamon has reverted to Labramon, who resembles Mei, the puppy that used to belong to the little girl the Tamers are all trying to protect. She doesn't know him until he says her name, and maybe ten minutes later, he dies in order to take down the movie's villain as she screams his name and cries.
Let's not forget the musical number from the dub version of "Runaway Locomon"
Promise that you'll stay for the sunset. And when the moon shines through the darkness, we can find the path that leads us home, and on the way you'll, maybe, sing me a song. Promise that you always will be there, hold my hand if ever I'm real scared. Help me stand up tall if I fall down. Make me laugh away all my bluest days. How could you promise you always would be there. Why'd you have to go away somewhere? Every morning and every night, do you watch over me like the sun in the sky? Am I all alone or standing in your light? I wish that I could, maybe... sing you a song tonight. ...You promised me we'd stay for the sunset...
Listening to the song while watching the scene with Rika and her dad really drives the point home.
Grani, especially his Dying Moment of Awesome. A tireless, reliable "vehicle" program that acquired sentience, came to understand the threat of the D-Reaper and what was at stake, was repurposed as Gallantmon's "steed," and when too badly damaged to continue, told Takato and Guilmon that he cared about them and merged with Gallantmon to enable its Crimson Mode. He was like a digital version of The Giving Tree...