After the great victory gained by humans against the Titans in episode 13, when the trainees were collecting dead bodies, Annie was apologizing to a dead soldier. Who was this? Was it Marco?
In wiki it is stated.
Despite her seeming callousness, she has expressed a level of guilt and shock, most notably when apologizing to a certain corpse after the battle of Trost
Various sources say it was Mina which may be true because;
Mina is shown sitting with Annie at the tables in the background of the first few episodes. That shows that they had some sort of friendship between them, as Annie is asocial and would have otherwise sat alone.
As counter argument: It is stated that Mina's head is bitten off but in the anime there is no such scene.
After Eren's leg is bitten off suddenly by one of the Titans, she and the other two attempt to kill the Titan that injured him. However, a group of Titans arrive out of no where and one of them kills Nac, catching her off-guard as another Titan grabs her 3D Maneuver Gear's wire, Mina killed anime Mina is caught by a Titan. knocking her over to the wall. When she realizes where she is, it is already too late for her to escape; she is grabbed by the Titan and her head is bitten off
According to the wiki for Episode 13 Primordial Desire it states that: "At the same time, Annie is also looking at a corpse of a soldier, apologizing to them." There is no name stated for the solider it was most likely just the face of a fellow solider that she recognized.
Jean finds and identifies Marco's corpse as he is assisting in the retrieval of the bodies. Marco's death is off-screen, so as far as for the anime the exact means of the death can not be identified.
Armin does manage to recognize Marco's 3D Maneuver during the attempt to Arrest Annie, when questioned she states that she "found it" so there is a potential that she may have done something that either led to his death or had a hand in his killing. ref(Marco Bott)
According to the recent release of volume 77, I think it was, its revealed that Annie did have a hand in Marco's death. After Marco overheard Bertholdt and Reiner's conversation about their true identities, he approached them and Reiner tackled Marco. After Annie came along, Reiner ordered her to take his 3DMG and leave him stranded. They watched a titan eat Marco in horror.
Concerning chapter 77, Reiner asked Annie to take Marco's 3D gear. She refused. Then Reiner manipulated her and forced her to do it by threatening her and her father. After much hesitation, she finally did it and she watched Marco being eaten. She cried a lot because Marco was being eaten.
Well, when it shows Jean talking to the lady who was asking about Marco, it shows a headless person on the roof. So I believe that was Mina. The person Annie was apologizing to was not Mina. Of course, it wasn't Marco either since he was bitten into. She could've even just have been apologizing to a random dead person.
If you read the manga, it does show how Reiner was forcing Annie to take of Marco's gear. Reiner, Bertl, and Annie were each saddened about Marco's death.
Even so, maybe Annie was apologizing to Marco. Reiner probably wanted her to take the body and dispose of it herself. She knew Jean would come by, knowing they were friends, so she propped it against a wall instead. That is my personal belief.
Annie has feelings too. It shows her crying over Marco in the Manga. Of course, she would do something like this to make sure Jean would know about Marco's death.
The corpse is shown to have 3DM gear attached which makes it impossible to be Marcos corpse as his gear was removed by Annie before his death. The hair on the corpse is dark and long, in the shape of a pigtail. It is very obvious that the corpse is Mina Carolina's.
Furthermore, Mina and Annie were illustrated to be very close as they sat together in the dining room, which would explain her apologizing to the corpse - she was her friend. Finally, Mina's head was bitten off which is shown in the image as well. If it was Marco's corpse it would be against the wall, like when Jean found him, but this corpse is on the floor, not against a wall, so it could not have been Marco.
I think she she is saying sorry to Marco, but she's substituting some cadets body for Marcos, as Annie would assume the Titan wouldn't leave a body.