The Doomsday Titan
Eren is best described as hardheaded, strong-willed, passionate, and impulsive, which are attributes of both his strong determination to protect mankind and eventually escape the Walls. As a young child, he was so intent on joining the Scout Regiment that he argued with and shouted at his mother, referring to the people in the village as "silly" and compared them to complacent livestock.
Eren cares deeply for Armin, his best and only friend before he met Mikasa, and most importantly, his family, risking harm and even death in order to help them. This quality was most often demonstrated in the form of Eren as a child taking on the larger boys who would bully Armin without hesitation; more tragically when he desperately attempted to lift the rubble crushing his mother during the Titans' assault in Shiganshina. In contrast, Eren is also capable of displaying very violent behavior in the defense of people he does not even know. This is highlighted when he brutally slaughtered two of the human traffickers who captured Mikasa in order to rescue her, despite the fact that they were total strangers at the time. Since witnessing his mother's gruesome demise and losing contact with his father, his fierce protectiveness to those he has left has only intensified.
Eren has a number of defining traits that date back to his childhood. He came to share Armin's awe of the lands beyond the Walls after the latter showed him a book depicting the wonders of the outside world, and decided to see it all for himself. This led him to develop a scornful attitude to anyone content to live and die within the confines of the Walls without ever setting foot outside. This very contempt may have been the reason he did not easily make friends with his peers. He resented both the Walls that "caged" them in like cattle and the Titans that had driven humanity to erect them in the first place. As a result of this, he greatly admired the soldiers of the Scout Regiment, regarding them as "heroes," and wishing to join their ranks as soon as he became eligible for enrollment. He easily took offense when anyone spoke badly of the Scout Regiment, and his determination to join their ranks caused friction between himself and his mother, as well as to Mikasa.
Eren has a marked lack of self-restraint that often led him into trouble, causing both his friends and family to worry about him. His limited self-control and reckless behavior extended to his temper, and as a result, it was short at best and murderous at the very worst. When sufficiently provoked, his anger could drive him into a frenzy, accompanied by horrific levels of violence. Like many children, he tended to view the world in terms of black-and-white, having great compassion for humanity and its plight, but deeming those individuals who deny others their freedom as worthless scum unfit to be left alive. He also had the habit of thinking with his heart rather than his head, even in situations where his own life was in danger.
These characteristic qualities have become more highly pronounced since his mother's death. He now focuses his destructive rage on the Titans, holding a virulent hatred for them, with the goal to rid the world of Titans one day. Fueled by vengeance, he vowed to personally wipe every single one of them out and restore mankind to its rightful place. Though he feels great remorse for quarreling with his mother till the very end, he still continues to go against her wishes by seeking entry to the Scout Regiment. So strong is his desire to exterminate the Titans, he was plunged into despair when it seemed he could not even master the basics of the omni-directional mobility gear during training when his failure was in fact due to faulty equipment. He is unafraid to voice his intentions of leaving the Walls to eliminate the Titans and is so consumed by his hatred that he seems not to fear them at all, earning him the nickname "suicidal bastard" from the other cadets. Though Eren spoke about eradicating the Titans with seemingly great confidence, his bravado hid deep insecurities.
He possesses great amounts of selflessness, perseverance, fortitude, and a vigorous determination; managing to graduate fifth in his class despite his lack of natural talent. He greatly appreciates and/or admires his comrades, and will swiftly fly into a rage if one of them is injured or killed in front of him. He prizes human life, but his personal sense of justice dictates that anyone who steals away the freedom and dreams of others deserves to die. Though his ordeals since the struggle for Trost have taught him restraint, Eren still tends to behave quite rashly and speaks his mind even when he knows he should not. This has put him in grave danger on numerous occasions, and he often finds himself having to be rescued.
Eren, however, tends to lose confidence in himself and blame his incompetence when confronted with how he could do little to nothing whenever something went wrong, believing a course of action he took - or failed to take - was the reason for the disastrous results. This includes when Eren put his trust in the Special Operations Squad when he could have fought the Female Titan himself, resulting in the deaths of all of his squadmates save for Levi, and for the deaths of the many veterans that perished in the rescue operation after he had been kidnapped by Reiner Braun and Bertholdt Hoover.
Eren's ordeals since the coup d'รฉtat have mellowed his personality considerably. When he discovered the truth about his father the night Wall Maria fell, he was willing to ask Historia Reiss to eat him for the good of humanity. He admitted that he viewed himself as weak, and that his reliance on the Attack Titan proved it. It took a lot of encouragement from his friends for him to overcome this state of despair and regain his passion.
By the time he finds out from his father's books that Titans are actually humans who were forced to transform by the Marleyan Government, he seems to have let go of his hatred against them. When passing by a slow-moving Titan en route to the ocean, he shows no signs of anger or unrest, and instead calmly calls the Titan a "fellow patriot."
Eren has been noted to be quite charismatic and has a talent for delivering impassioned speeches, often unwittingly managing to persuade the mass of his listeners to sympathize with his point of view. Even Jean Kirschtein, his bitter rival, has admitted this, as he convinced over half a dozen recruits to consider joining the Scout Regiment before the Battle of Trost. Four years later, he makes use of this skill consciously, as shown when he easily lifts Falco's spirits and convinces him to deliver a letter outside of Liberio, despite the fact that Eldians are usually forbidden from leaving the internment zone.
In the years since the recovery of Shiganshina, Eren has shown remarkable progress in his analytical and tactical skills, both in and outside of battle. Apart from maintaining his cover as a Marleyan soldier for an extended period of time and procuring a method of transporting his letters to the Scout Regiment, Eren accurately guesses the source of Falco Grice's injuries and the reason why he wanted to become the next Armored Titan. Later on, he successfully leads Reiner into a trap, and in his battle against Lara Tybur, he quickly deduces the true location of the War Hammer Titan. When he is unable to crack the crystal encasing her with his Attack Titan's jaws, he is resourceful enough to use the Jaw Titan's jaws as an improvised nutcracker to kill her and obtain the War Hammer Titan.
Eren is aided in this by his unnervingly stoic and calm demeanor he developed since infiltrating Marley, never losing his composure and usually wearing a blank expression on his face, no matter how dire or stressful the situation may be. Although he admits to becoming angry when Koslow traumatized Eldian soldiers, he remains outwardly unfazed during the incident. Additionally, he appears to have become less proud overall, thanking Mikasa for helping him, which he normally used to dislike and criticize her for.
His infiltration of Marley has given him a change in perspective regarding his enemies. After spending time as a Marleyan soldier, he comes to understand that his enemies are people no different from his own. Because of this, he deeply empathizes with Reiner and the other Warriors who were sent to Paradis Island, letting go of his desire to kill them in revenge for attacking his home and stating that they are simply victims. Nevertheless, he still prioritizes his personal mission of destroying Paradis' enemies for the sake of his people's freedom, regardless of how many lives are lost on either side. In spite of this, he seems to have developed a dislike for war, referring to it as "hell." He believes that, with good reason, no one would want to go to war if they knew the suffering it would put them through.
Eren believes that righteous goals sometimes require unethical means. He is ready and willing to use such methods himself when he declares war against Marley. Eren abuses the twelve-year-old Falco's trust, fully aware of his family's precarious situation, and eventually kills dozens of innocent civilians (including children) as collateral damage in a surprise attack on Willy Tybur.
Whilst assuming his Titan form, Eren seems to become more savage, bloodthirsty, and all-around uncontrollable. Though he is able to consciously control his Titan body after the struggle for Trost, he can very easily become caught up in the pleasure of slaughtering his enemies, smiling as he prepares to torture or kill them, and indulging in thoughts so brutal he gives the impression of being insane.