I think the reason everyone forgets Ymir is that she is just a nothing character that literally just exists as a child-shaped plot device. She literally has no traits beyond what she is rather than who she is.
Eren is just a fucking retarded loser who Isayama tried to make sympathetic in the worst way possible. That whole "Just a teenager" excuse does not work since Eren never really acts like a normal teenager at all in the timeskip and is never humanized for the most part.
As a character he fails since in the timeskip era he is portrayed as this super calculating mastermind with a masterplan. But when you look at the shit he does and his plan as stated in the manga, it's a trash plan that is ultimately selfish yet treated like it's altruistic. When you look deeper it gets worse since he basically doesn't bother to take any actual action, never tries to put effort into changing fate all because Mikasa pretty much family-zoned him which was justified given he rejected her advances when they were younger and she simply moved on. This apparently equals that fate can't be changed so Eren deliberately fucks over his people, and then proceeds to destroy the world just because. Though the reality was simply Isayama just drawing out his own autistic fantasies of him crushing his bullies.
I mean, true. But I'm not going to forget that.
No, it isn't. Isayama completely fucked up by focusing on Eren as the protagonist and just not making him clearly pathetic enough. I think a large part of the problem is that NO ONE is humanized. The plot itself, to me, just gets completely emotionless after the timeskip (except when Sasha gets killed). Besides that, there's no real pathos anymore. The story is just meandering waiting for Eren to do something, fucking anything. And there's this massive build up to this super plan and then just....nothing. It turns out there is no plan, he was just going to kill everyone because fuck you, that's why.
And it is hysterical that basically, every single thing Eren wants to happen does (except when he loses in the end). That's another problem with making Eren 'pathetic'. Everything he wants is succeeding until it doesn't anymore.
Attack on Titan is the most gratuitously indulgent example of author fiat. Things happen because Isayama
wants them to happen. There's no story logic or story consequence. Things just happen because 'fuck you, that's why'. Gabby finding Eren and perfectly blowing his head off to have his brother catch his head in his hands? KEIKU. It turns out he wasn't doing anything at fucking all and all that waiting on Eren to do something was just a waste of fucking time. 90% of AoT is a waste of time. The first half is a waste of time because there's a complete tonal shift and the titans as a threat or mystery are completely eliminated. The timeskip is a waste because the majority of it is waiting for Eren to move the plot while everyone else does inconsequential shit.
I think Erenfags are just aggressively coping. I was never a characterfag (plotchads where you at?), but I will say after the timeskip Eren was probably the most interesting and engaging. The protagonist going off the rails and becoming the antagonist has spectacular potential. For a while, Eren was a genuinely great character. I still maintain that up to a point, the Eren/Historia ship (god, I hate shipping) was based off faith in Isayama. Why would he show us all these scenes? Why would he include these "hints" ? Why would he just have a character an entire arc was about putting on the throne fuck a former bully farmer? To many it made no fucking sense. Isayama is a genius right? This must all be for a reason, surely? Back then, I saw where they were coming from.
The issue is Isayama was revealed to either want to "hurt the reader", enjoy being an absolute retard or any other of the many theories for shitting up his manga spectacularly. I think Erenfags just got too invested in their narratives and predictions through the 3+ years or so of theorizing and discussing this shit month after month.
Most of them don't even internalize what the final 20-25 chapters are saying or mean for Eren's character. They're still seeing his "potential" and their headcanon. They're delusional. They're coping. Isayama's ending has unironically driven lots of people to maximum cope. The manga ended so shittily they literally can't absorb it. Full on denial of the reality of who Eren is according to the mangaka. Isayama broke them by making Eren what he is.
Plotchads? What fucking plot lol. The plot dies after the timeskip. Shit, the plot dies as soon as they reach the basement and EVERYTHING is exposition dumped at them. Fuck mysteries, just dump all this shit on you.
I completely don't agree with that. Post-Timeskip relied entirely on Eren moving the plot at the pace of a fucking glacier. The protagonist becoming the antagonist is difficult to pull off. You have to remain sympathetic to their motives and ideals. Isayama only fake does this by drawing out Eren's master plan of simply 'murder everything' and doing everything in his power to create false foreshadowing and threads he intentionally drops because they would be inconvenient to where he wants the plot to go.
My honest opinion is that Isayama had absolutely no idea where he wanted to go with this. He got bored of the horror angle. He used Eren to completely nuke it with tree guillotine and then finally gutted it with Expo dump. After the basement, since he'd thoroughly fucked the plot, he used on of the worst writing devices to cover his fuck-up: The timeskip. Here he could 'hide' information to the reader (IE: Buy time to figure out what the fuck he was going to do).
He just kept buying time by introducing thread after thread and shit that wasn't connected. He dragged it out for as long as he could and then when he couldn't, he went 'lol no plan' and threw everything out the window. The 'hurt the reader' excuse was just pretentious bullshit hiding his own autism and incompetence.
Though he certainly did fucking break people. Good lord.
Honestly I could buy him ruining Eren on purpose. The vast majority of what we shit on him for IS from the final chapter. Most of that wasn't present before and it ruins Armin and a large amount of the plot too. I don't think it was an accident either, and it wasn't just edge for the sake of edge that Eren was suddenly responsible for his own mother's death. His character got really erratic after Liberio since Isayama clearly didn't know what to do with him, even during Liberio he barely talked and seemed to be thinking things out still, like Isayama. The mystery to his character at that point was Isayama buying time.
That said he definitely wanted Eren to be pathetic from the beginning, he specifically told the anime studio to make him more so and then we got that awful scene where Eren tries for ten minutes to transform and fight Annie. In the manga he turned out sympathetic instead for the first few dozen chapters if you like justifiably angry revenge protagonists. His faults at the time get washed over by "but his mom got killed and titans are bad" and Isayama's idea of pathetic up till that point was "angry, and resentful that he can't do anything about it" which just makes the reader sympathize harder. He must see something in himself that Eren shares that disgusts him, or had the potential to overcome when he couldn't, and having anyone be a fan of the guy probably freaks him out. He just wasn't able to communicate that part of Eren consistently. Instead it feels like we have several different Erens, and that angry Eren people liked was just Isayama ripping off a few other angry protagonists (hell just read a summary of Kamille Bidan in Zeta Gundam). And since he only had one chapter left, he had to go in for the kill. That's why he had Eren kill his mom himself, he had to destroy Eren's basic motivation so the reader couldn't possibly sympathize anymore. And then there's his out of nowhere lust for Mikasa.
If I had to come up with an exact origin here, I'd say Isayama was bullied as a teenager and was really pissed off about Japan's post-wwii treatment, and loathes the person he was back then. He doesn't want to be Eren anymore, it was never fun and he can't make bad things go away with a titan like Eren can, he wants to be Armin, he wants the readers to like Armin and idolize him instead. The readers just wanted to see Eren get mad and kill titans. Now anyone with sense thinks both of them suck. Good job Yams.
I think he ruined him because his editors told him 'no, you can't have a sympathetic genocidal maniac you stupid fucking autist'. So he went out of his way to make everything extraordinarily pathetic and ruin him at the very last second.
Maybe he wanted him to be pathetic from the beginning. If so, he did a TERRIBLE fucking job of it. Since the plot relies on him a lot, there's no way to do it because Eren has to move the plot along. And like you said, Eren is basically schizophrenic as a character at this point.
If you look at it, everything Eren does works until it doesn't. Then that shifts to Armin, except everything works because it has to because Eren has to lose. There's really no rhyme or reason to Eren losing, because Isayama completely fucked up Eren losing. There was honestly no in-plot way for him to lose so he had to magic it so he did. Its really noticeable.
And its not hard to see why a lot of people cope with Eren, because there's literally no evidence for anything until the last 20 or so odd chapters where the story is twisting in knots to justify itself and just gets worse and worse as the ending comes.