WARNING: The following contains spoilers for Season 4, Episode 7 of Attack on Titan, "Assault," now streaming on Crunchyroll, Funimation, Amazon Prime and Hulu.
The Survey Corps' attack on Marley really comes to a head in Season 4, Episode 7 of Attack On Titan, wherein we see the new and improved regiment as they defend Eren and help take down the Marleyan Shifter Titans. But how did they manage it? In Season 4, Episode 5 "Declaration of War," Willy Tybur mentions to Theo Magath that he suspects a betrayal in the Marley military, as Eren and his friends would need help getting into Marley.
Only a high-ranking military official could possibly facilitate their invasion. In the latest episode, "Assault," we get a brief glimpse of a new character that could be the potential Marleyan traitor.
Episode 7 introduces a new member of the Survey Corps: Onyankopon. While Eren battles the Jaw, Beast, War Hammer and Cart Titan on the ground in Liberio, an airship starts to fly by to collect the Scouts and take them back to Paradis in a well-planned retreat. It's in this scene that we meet Onyankopon, who steers the airship. He's definitely a new face, both in the anime and in the Survey Corps. But based on his different outfit, he may only be an associate of the Survey Corps, not an actual member. He doesn't wear the ODM gear nor does he have the Survey Corps' famous logo anywhere on his body.
Could this be the Eldian conspirator? There's another controversial detail that suggests it's possible. From what we've seen of Paradis Island, its residents are very homogenous, possibly deliberately so. Everyone looks Caucasian and has mostly Germanic names and culture, and the Eldian race itself takes real-life inspiration from people of Jewish descent. Onyankopon is a person of color, which doesn't mean he absolutely can't be from Paradis, but we haven't seen any other people of color in Paradis throughout Attack On Titan, which is problematic in and of itself.
Given the four-year time gap and the knowledge that there a conspirator from Marley does indeed exist, we can definitely speculate it could be Onyankopon, as we would any new face who suddenly allied themselves to the Corps without a Scout uniform, to be honest. Who knows what's happened in the four years since the end of Season 3? Certain citizens of Marley, especially high-ranking ones that would know how to pilot an airship (pretty advanced technology for the citizens of Paradis), could be privy to information regarding the Eldians and the truth about their history. This could potentially garner sympathy for Eren's cause and justify a betrayal of Marley's old way of life.
The truth of Eldia and its people became known to the citizens of Paradis at the end of Season 3. Now, it is certainly possible for them to catch up to the modern technology that the rest of the world enjoys, but not without outside help. For them to either build an airship or have access to one requires someone with the know-how and the expertise on the subject. Clearly, the man with the steering wheel in his hands is the man for the job. So, while we can certainly speculate on who the conspirator is, we still don't know why. If it is Onyankopon, why did he betray his own people to help the Eldians?
The final season of Attack On Titan is rapidly increasing both the speed and the stakes of the story, and yet there are still so many unanswered questions. All we know for certain is that we will see more of Onyankopon and the Survey Corps together, at least into the next episode. Hopefully, his role and history will be expanded upon so that we can get a better sense of who he really is and why he's being introduced so late in the anime. Marley's traitor is still out there and until Marley finds out who it is, Eren and the Survey Corps will have a major tactical advantage.