#Marco yelling 'Jean! Look out!' when Gabi gets inside the airship and Jean looks towards the entrance but is too late-

[twpsyn-who]

Soulmates AU in which your soul is bounded to your soulmate's soul.

What does that mean exactly? That when you die, if your soulmate is still alive you can't move on. Instead your soul is stuck by your soulmate's side until they die too, so the two souls can become one in death. Depending on how strong the bond between the two is, one could see (and even interact!) with their deceased soulmate.

Now this but, you know by now, JeanMarco. With Marco waking up and the first thing he saw was Jean's terrified face while looking at the dead body- Marco's half eaten dead body. He learns it pretty quickly that he can't leave Jean's side, bound to follow his best friend around. He also learns that is possible for Jean to see and hear him during the ceremony of burning the fallen soldiers during Trost ; it lasted so little, too little for all the things Marco wanted to say. If he knew Jean was listening from the beginning, he would've started with exposing Reiner and Berthold right away. By the time he took notice that not only Jean was looking at him, but listening too, it was too late. Every other word from him was meet with silence, unheard by the living.

Isn't until the 57th expedition outside the walls that Marco learns that he can interact with Jean too, touch him. In his panic after Jean's failed attack on the Female Titan Marco didn't think and went straight to his best friend's side, touching his shoulder while looking for any injuries. Jean's eyes went straight to the place Marco's was touching, looking confused- until he followed the arm and saw Marco in front of his very own eyes. And just like everything, it lasted too little for Marco's liking. The Female Titan took hold of Reiner, crushing him in her hand, and Jean could no longer see him.

It was later, much much later than either of them would've liked, for Marco to understand how everything works. To know that he could only interact with Jean when his mind was filled with thoughts of Marco. By then Jean already began to move on, the secret of Reiner and Berthold's true selves already out. So Marco could only watch, unable to be there for his other half- for Jean.

And it was when they saw the ocean for the first time that Marco understood why he was truly stuck by Jean's side. When he saw his best friend having fun with the others and realized just how much he loves him. When he realizes that all he truly wants is to be by Jean's side, the way they promised each other to do during their training days.

Edit : Think of Ben and Klaus Hargreeves for easier visual.