WARNING: The following contains minor spoilers for the Attack on Titan manga, available in English from Kodansha.
Season 4 of Attack on Titan will see the return of long-established protagonists and antagonists, however, the trailer for the final season has also introduced a slew of new characters. If the anime stays true to the manga, which it most likely will, then several of these characters will take on major roles during this season, including the following.
Gabi Braun
Along with going to Reiner's hometown for the first time, audiences will meet the antagonist's family, including his young cousin Gabi, who lives in the Liberio internment zone. Similar to Reiner, Gabi and her family believe in the end of Eldia, which leads to her enlisting as a Warrior candidate.
Being a warrior candidate means being a child soldier and training to become one of the titan shifters. This program is what led Bertholdt, Annie, Reiner, Porco, Marcel, Pieck, and Zeke to become shifters at a young age, and Gabi intends to carry on the family legacy, even if it means she inherits her cousin's titan, the Armored Titan.
Porco Galliard
Porco, while a warrior, did not inherit his titan abilities until Reiner and Bertholdt brought Ymir to Marley. It was alluded in Ymir's letter to Historia in Season 3, Episode 21, "Attack Titan," that she would be killed, and the flashback saw her being chained up like a sacrifice for someone. With the re-introduction of the Jaw Titan, it's clear that Ymir was eaten, and her abilities were passed down to someone else.
This someone is Porco, who happens to be the younger brother of Marcel, the Warrior Ymir ate to inherit the Jaw Titan. Now with his brother's titan, Porco fights alongside the Cart Titan and the Marleyan military, proving to be incredibly knowledgeable about its capabilities, which can be marked up to the fact that he's trained for this kind of position since he was a child. Along with easily tearing apart train tracks, Porco also shows how agile he is as the Jaw Titan.
Pieck Finger
Pieck technically was introduced in Season 3 as the Cart Titan, however, Season 4 will be the first time viewers will get to see her in her human form. Unlike Marcel, Annie, Bertholdt and Reiner, Pieck did not go to the island, most likely because her titan serves the Marleyan military better.
The Cart Titan, as seen in the trailer, can be equipped with guns, turning Pieck into a makeshift tank that can maneuver better than any vehicle. Along with this, Pieck is one of the best all-around Warriors given her intellect and skills.
Falco Grice
Falco is another Warrior candidate, and unlike Gabi's introduction, which shows the girl excited to be at war, Falco is already beaten down. He's badly injured and requires the help of his older brother Colt, the eldest Warrior candidate, to get to safety.
It's not too much of a surprise to see Falco in this position because he is one of the physically weaker candidates, but what he lacks in strength, he makes up for in compassion and drive. Falco is also one of the few Eldians within Marley who is not completely disillusioned, making him Gabi's foil for the season.
The War Hammer Titan
There are nine titan shifters in Attack on Titan, and audiences have met at least one iteration of eight of these titans. Season 4 is introducing the last of the nine titans with the War Hammer Titan. As the trailer alludes to, the War Hammer Titan is going against Eren's titan, so it's fair to assume she aligns herself with Marley.
While most of the other shifters are covered in skin or have their muscles exposed, the War Hammer Titan is covered in a white substance, which seems to act as an advanced version of the Armored Titan's protection. What further makes the War Hammer Titan stand out is the fact that she can generate a massive hammer from her body. If her body, and in turn her weapons, are made of the titan crystal material the Jaw, Female, and Attack Titan have all used, then she could be one of the most dangerous titans with her mastery of this skill.
Willy Tybur
Willy Tybur is not seen in action like the other characters on this list, befitting his character in the manga. As another Eldian born into Marley, he has also consumed a lot of the propaganda against his people, and this has led him to hate his bloodline and wish for the extinction of Eldians. However, he does not want to die because, like Eren, he was born into this world.
As one of the few well-off Eldians in Marley, holding a certain degree of political power, he carries himself as a charming, well-groomed, and charismatic man who speaks on behalf of the Eldians living within Marley. He also stays out of the public eye as often as possible, but when he is out publicly, he is treated with fanfare, as the Eldians adore him, even though he is in support of their race's eradication.
Yelena
Yelena is seen the least in the trailer, which coincidentally stays true to her character in the manga. She keeps her secrets close to her chest as a loyal agent who's driven more so by personal goals than the agenda of Marley or Eldia.
However, the small clip of her in the trailer manages to capture a huge part of her personality. Given how devoted she is, it's no surprise to see Yelena basking in the destruction of presumably her enemies.
Attack on Titan Season 4 is scheduled to release in this fall.