WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 24, "Pride," now streaming on Crunchyroll, Funimation and Hulu.
As a dark fantasy-action series set in a post-apocalyptic world, Attack on Titan is certainly no stranger to dishing out tragedies. With each passing season, the death toll rises parallel to the presence of grief and sorrow as familiar faces disappear one after the other, often in horrific ways. Living among great devastation usually prompts the existential question: what did they die for?
This question has been explored before in Attack on Titan, specifically by Erwin Smith who, in the heated battle to reclaim Wall Maria, exclaims to his soldiers that it is them -- the living -- who give meaning to the deaths of their comrades. The hopes and dreams of these fallen soldiers are still alive and propel the strength and motivations of those left behind to keep moving forward. That being said, one early death continues to impact the Scouts seasons later -- especially Jean Kirstein.
In Season 4 Episode 24, Jean is offered everything he's ever wanted -- to live a relatively safe and easy life -- by joining the Jaegerists. He grapples with this decision before deciding to join Floch Forster, but at the end of the episode Jean betrays them and sides with his friends. By doing so, he's thrown away his previous ideals and embarked on the impossible, joining this new team of both Eldians and Marleyans who are prepared to risk it all to stand against Eren Jaeger. When asked why, Jean mentions ashy bones, a clear reference to an old friend and former cadet, Marco Bodt.
Marco was a member of the 104th training corps and originally thought to be another casualty of war. A skilled soldier who graduated as the 7th highest-ranking trainee in his class, Marco is remembered for being intuitive and possessing the natural qualities of a leader, including habitually raising his comrades' morale. His idealistic personality was a direct contrast to his best friend Jean, who was introduced as being hot-headed and brash, but they balanced each other out well. It was Marco who saw through Jean's flaws and recognized he had the capabilities to be a strong and reliable leader -- even better than Marco himself -- and he encouraged Jean to be more than he is.
Following Marco's death in Attack on Titan, Jean takes his friend's words to heart. Instead of joining the Military Police Brigade like he planned, he joins the Survey Corps despite being terrified to the core. Jean changes his selfish ways as the memory of Marco motivates him to keep fighting, even amid the horrors and devastation surrounding humanity.
The full circumstances of Marco's death aren't revealed until a flashback in Season 3. During the Battle of Trost, Reiner Braun and Bertholdt Hoover discuss their titan forms and how they breached the walls twice -- a conversation Marco unfortunately stumbles upon. From their discussion, he works out that there are humans other than Eren who can turn into titans and that these traitors are hiding in plain sight and working against Paradis. Reiner attacks, but unable to let Marco live after discovering their secret, fellow titan shifter Annie Leonhart removes Marco's ODM gear, leaving him defenseless to the titans and pleading for a chance to talk things out as he's eaten alive.
However, indirectly murdering their friend is not without consequences. Horrified by the situation, Annie hesitates, clearly reluctant to take part, but gives in following excessive prompting from Reiner. She watches his demise with tears and continues to be haunted by it through nightmares. This event likely contributed to her mental break at the end of Attack on Titan Season 1.
This is also the point where Reiner's grasp on his identity becomes fragmented, asking why Marco is being eaten despite leaving him for dead mere moments ago. This problem persists as Reiner's mental health deteriorates drastically over time, often growing confused as to whether he is a Marleyan warrior or a soldier for the Scouts. It's clear that Marco's death doesn't weigh heavily on just Jean.
Marco's influence continues to live on through Jean, who exclaims that if he had chosen to stay with the Jaegerists or even cooped up in his room, Marco would never have forgiven him. Now, Jean continues to rise above the easy way of life in spite of his own fears. His betrayal of the Jaegerists is directly attributed to his friendship with Marco, allowing him to save Yelena and Onyankopon, buy Mikasa Ackerman and co. time to escape and earn the trust of Pieck Finger and Theo Magath. In turn, Marco has impacted the entire series as Jean's actions mark a major turning point for the Scouts. And just as Erwin proclaimed, Jean has ensured that Marco's death was not in vain.
