There were many, many deaths during the Attack of the Giants. Hajime Isayama has never spared himself in giving deaths to the right and left both among the extras and between the main and secondary characters. And many of these have marked turning points in the lives of different individuals.
So dig up the dead of this millennial war caused by the clashes between Eldia and Marley creates hatred and bitterness. It is one of the cases addressed by chapter 127 of The Attack of the Giants that puts Reiner Braun and Jean Kirschstein in a difficult situation. During the unpleasant camping that was held overnight before the end, first there was an almost clash between Annie and Mikasa, after Yelena she managed to stir the spirits by making the name of Marco.
Marco Bodt he was one of Jean's best friends and the leader, as far as known to the Legion, was the whole group of Marleyan warriors. However, during the discussion, Reiner was unable to stop saying that it was his fault that Marco died having been the Armored Giant who ordered Annie to remove the equipment, while he threw him on a roof and he kept locked to prevent him from running away.
Reiner then raised the dose by admitting that he could not understand why Marco was being eaten, fueling doubts about his psychic state of the time. Unfortunately, Jean was unable to resist and he started hitting his ex-comrade as hard as I can. Despite this rush of fury and anger, Jean will still decide to join the group that must try to save the world, admitting that in any case he will never forgive Reiner for what he has done.
Now the protagonists of The attack of the Giants they have another enemy to deal with, and he's a human from Eldia.