Yelena is a villain in the final story arcs of the manga Attack on Titan, first appearing in the Marley-arc.
Past
Yelena was raised in a nation that was eventually conquered by Marley. Under Marley's regime, she was conscripted into the Marleyan military where her life was saved by the Beast Titan during a naval battle. Looking up to the Titan and its possessor, Zeke Yeager, as a god-like figure she was the first to join the fraction eventually to be known as the Yeagerists.
Convinced that Marley would need to be stopped and that Zeke's extreme plan of sterilizing all Eldians in order for no more potential Titans to be born, Yelena assisted Zeke in amassing followers - mostly soldiers from colonies occupied by Marley. Fiercely loyal and devoted to Zeke, Yelena would kill anybody, even close companions, if they endangered her end goal. Yelena and several others were eventually sent to Paradis Island by Zeke, where they were to gain the trust of the inhabitants in order to further Zeke's plans. However, their prime task was to make contact to Eren Yeager as well as to secretly poison the military leaders of Paradis with wine spiked with Zeke's spinal fluid, which would allow Zeke to transform them all into Titans via scream.
In the story
While onboard the survey ship tasked with patrolling the border of Paradis, Yelena and everybody else are captured when Eren Yeager - as the Attack Titan - grabs the ship and heaves it onto the land. The soldiers are confronted by the survey corps, only to reveal their true colors as defectors when Yelena promptly shoots her own commander. The soldiers are captured nonetheless and Yelena as well as another soldier, Onyankopon, are interrogated by Levi Ackermann and Hanji Zoe. They inform the two that Zeke has sent them to help the island advance in order to be able to become a global power so that they would eventually be able to get revenge on Marley. Despite the mistrust of the islanders, the defectors - commonly known as the Anti-Marley-Volunteers - are allowed to assist the people of the island. They bring several technical and military innovations and help the island advance.
However, Yelena also secretly makes contact with Eren Yeager, Floch Forster and several others. By revealing some pivotal information to them, Yelena brings them into the fold, forming the Yeagerists on Paradis Island. The group amasses several followers believing that Eren with his Titan Powers should be the ones to lead Paradis Island in the battle against Marley. However, the group remains relatively secret, biding their time and using several new restaurants built by the Anti-Marley-Volunteers to poison the military brass with the spinal fluid wine. Eventually, in order to deal a crippling blow to Marley as well as to bring Zeke to Paradis Island, Eren secretly invades Marley, forcing the Survey Corps to follow him in order to make sure the Founding Titan remains safe. Yelena and Onyankopon join the Survey Corps in this mission and Yelena presumably makes contact with Zeke back in Marley.
From him, she gets the order to keep two of Marley's titan shifters, Pieck and Galliard, away from the festival in Liberio, since this is where the attack will start. Disguised as a male soldier with a fake beard, Yelena approaches Zeke, Pieck and Galliard before the festival and claims that they need to follow her to General Magath. After sending Zeke off, Yelena leads the other two shifters into a room and switly opens a trap door, causing the two to drop into a hole. When the attack on Liberio starts, Yelena manages to regroup with the Survey Corps and enters the escape blimp, where she is present when Eren, Zeke and the rest of the Survey Corps returns.
Back on Paradis, the military investigates Eren's actions and as a result, Yelena and the other Anti-Marley-Volunteers are put under arrest. Yelena cooperates fully, which makes Commander Dot Pixis even more suspicious. Despite Pixis' best efforts, it is too late since the Yeagerists already control most of the military due to the threat of turning them all into Titans. Thus, the Yeagerists are able to overthrow the military and to install themselves as the de-facto leaders of Paradis. After Eren and Floch arrest most of the rest of the Survey Corps veterans, Yelena visits them in their cells. After one of the Yeagerists insults the memory of the recently deceased Sasha Braus, Yelena coldly shoots him in the head and apologizes to her prisoners.
She promises to reveal everything to them, detailing how Zeke's sterilization plan is supposed to work and apparently touching Armin - who presumably fakes these feelings to goad her into a sense of security - but is then informed that an enemy Titan Shifter has infiltrated the military. When Yelena and Onyankopon hurry to Eren's side, he as already incapacitated Pieck who apparently plans to defect and join the Yeagerists. Yelena urges Eren not to trust her and although Eren is careful when dealing with Pieck, he walks into an ambush and - to the horror of Yelena - ist attacked by Galliard in his Jaw Titan form while Marley's military arrives in several airships above Shiganshina.