The recent chapter of “Attack on Titan” has given readers a glimpse of what Marley is going through right now. But because the time skip is a bit confusing, the timeline's not clear yet. Now, fans know how Reiner, Annie, and Bertholdt became titans.
The Story Behind The Three Warriors
Annie, Reiner, and Bertholdt were three of the titan shifters introduced early in the manga and they are the Female Titan, Armored Titan, and Colossal Titan, respectively. The said titans are part of the nine original titan shifters. At first, it was perceived that they were in cahoots with the mindless titans when they breached Wall Rose a few years ago.
But it was revealed that they were actually Marley’s soldiers, warriors, in particular, is what Marley calls those who become titan shifters. It is most likely that these three were either forced or brainwashed to take part in Marley’s invasion of Paradis Island. Marley actually carefully chooses their warriors from the few Eldians they have kept locked up in their city.
As revealed in the earlier chapters of “Attack of Titan” manga, only Eldians can transform into titans and inherit the powers of the titan shifter so Marley trains selected Eldians at a young age. Marley makes them believe that their invasion is something that would save humanity. A good example is Grisha Yeager’s first son, Zeke, who betrayed him and the rebellion and became Marley's pawn, even turning into the Beast Titan.
Reiner and Bertholdt may have voluntarily joined as Marley’s warriors since they were a bit ruthless in their attacks against the Survey Corps and the walled city in Paradis. Annie, on the other hand, might have been forced into it because of her father, who is presumably being held captive by the Marley government. Annie has also shown in the first few chapters how she was against the killing of Marco and even saved Connie from a titan’s attack.
Marley Prepares New Warriors
Marley is actually preparing a new set of warriors to inherit the powers of the titan shifters that are currently in their hands. As revealed in “Attack on Titan” Chapter 91, Marley uses the war to determine among the Eldian youngsters who will get the Colossal Titan’s power. A new character was introduced, named Gabi and she has shown gusto in being chosen.
Reiner is presumably still in tatters since his last fight with Eren so Marley has probably decided to sacrifice him early to pass on the Colossal Titan’s power. In the latest chapters, Marley’s brainwashing is evident in Gabi’s thoughts since has referred to her fellow Eldians who ran away to Paradis Island as demons who need to be killed. This coincides with Reiner’s confession before that he, Bertholdt, and his other comrade, who was eaten by Ymir, were children who were not aware of what was happening.