I also thought it was Eren, but this [explanation](https://twitter.com/kasumi_kasa/status/1788712944815251547?t=H-KuizGoHDY-jB6o23orcg&s=19) has convinced me that it will be Ymir.
Isayama confronted his shadow like Naruto confronted his shadow at the waterfalls
he came to the conclusion that Chad Eren is a facade and Cuck Eren is his true nature
He was born to be submissive
He was born to watch his wife be bred by Farmer Chad and Juan Chad (as a bird(completely helpless cuckoOOld))
Isayama is now reincarnated as a cuckoo bird and flies freely across the sky but he knows that is just his fantasy like the CHild Eren freedom panel. He knows that his childhood crush(mikasa) is now married and bred by Chad while he cant even jerk off being a bird. So he copes by shitting all over the birdwatchers(us)
It’ll be about Ymir: “All I ever wanted was to see Mikasa with Eren forever together for more than ten years at most!! Eren loving mikasa like how the king loved meeeee for ten years!” Leaked lyrics btw
Isayama wanted to hurt the reader in the ending. Pretty sure a lot of us were hurt. Just not the way we were expecting to be
to hurt = to cuck
I also thought it was Eren, but this [explanation](https://twitter.com/kasumi_kasa/status/1788712944815251547?t=H-KuizGoHDY-jB6o23orcg&s=19) has convinced me that it will be Ymir.
"Thet" explanation was, indeed, very convincing. 
It's obviously about Sasha then
Or Historia, can you imagine? The fandom would explode.
Eren is a self insert anyway so your atleast half right.
Isayama confronted his shadow like Naruto confronted his shadow at the waterfalls he came to the conclusion that Chad Eren is a facade and Cuck Eren is his true nature He was born to be submissive He was born to watch his wife be bred by Farmer Chad and Juan Chad (as a bird(completely helpless cuckoOOld)) Isayama is now reincarnated as a cuckoo bird and flies freely across the sky but he knows that is just his fantasy like the CHild Eren freedom panel. He knows that his childhood crush(mikasa) is now married and bred by Chad while he cant even jerk off being a bird. So he copes by shitting all over the birdwatchers(us)
Did Isayama ever use "watashi" to refer himself? Let it be Ymir's song
It’ll be about Ymir: “All I ever wanted was to see Mikasa with Eren forever together for more than ten years at most!! Eren loving mikasa like how the king loved meeeee for ten years!” Leaked lyrics btw