.

The weird skeletal look isn’t something Eren deliberately chose for drama or style—it’s a direct consequence of how he unlocked the full Founding Titan power. Right before he made contact with Zeke and reached Ymir in the Paths, Gabi blew his head off with that anti-Titan rifle. Because his head and body were already separated, the Source of All Living Matter (that glowing centipede thing) had to massively expand the spine and ribcage structure just to reconnect them and form a usable body. That’s why you get this elongated, puppet-like spine with the ribs arching overhead and the head hanging down on long tendrils instead of a more conventional humanoid Titan. Had he still been intact when he gained full control, he almost certainly would have manifested something closer to Ymir Fritz’s original Founding Titan form (the more muscular, skeletal-but-still-coherent one from the flashbacks). The awkward, almost clumsy appearance ends up fitting the moment thematically though—Eren is no longer operating as a normal shifter. He’s become a pure conduit for the Paths and the Rumbling itself, less a person in a Titan body and more a walking apocalypse held together by bone and will. It looks unnatural because the situation was unnatural.