Ymir has demonstrated significant amounts of speed and agility in her Titan form. Though her Titan is relatively small and weak, its mobility excels in terrain such as Forests of Giant Trees. Her Titan also possesses an incredible biting strength which can be used with devastating efficiency against both Titans.
Over sixty years ago, Ymir was transformed into a Titan when she was exiled from her hometown. After wandering beyond the Walls seemingly stuck in her Titan form, her Titan came across the Warriors traveling to begin the destruction of the Walls. After catching and eating Marcel, Ymir returned to her human form and began a new life within the Walls.
When their commanding officers die confronting the Titans, Ymir speaks with Christa, reminding her of a promise they made to each other during their training days. Taking a knife from Conny, Ymir slits her palm and transforms to fight the Titans at the base of the castle.
In her Titan form, Ymir begins to slaughter the Titans around Utgard with ease, using claws on her hands and feet to slash at napes as well as making use of her fanged teeth. After a short while, Ymir's Titan is outnumbered and overwhelmed and nearly breaks the structure of the tower. Rather than risking the castle's collapse, Ymir allows for her Titan to fall into the horde until Christa calls out to her, telling her that she should destroy the castle if that is what it takes to survive the Titan horde.
Ymir then starts tearing away at the tower, using bricks and stone to fend off the Titans until the tower begins to collapse. Climbing the tower, Ymir tells the other recruits to grab onto her Titan, which protects the others from falling to their deaths.
When the tower is reduced to rubble, Ymir continues to fight the Titans, but she is soon outnumbered, and the horde eats away at her Titan until the Scout Regiment arrives to finish them all off. Ymir is extracted from her Titan, missing some limbs and with serious abdominal injuries, and Christa tells Ymir her true name "Historia" before Ymir falls unconscious.
The next time Ymir uses her Titan is after she agrees to help Reiner and Bertholdt in exchange for Historia's safety. While in a forest she transforms to face the Scout Regiment, which has come to retrieve Eren, who was kidnapped by Reiner and Bertholdt.Because Conny has seen her in her Titan form before, he prevents the others from attacking her, giving Ymir the time she needs to look around at the faces of those present. When she spots Historia, she opens her mouth wide and captures her inside.
Ymir then turns around and swings from tree to tree to escape. She meets up with Reiner and Bertholdt, who are waiting for her at the edge of the forest. On seeing her, Reiner transforms into the Armored Titan and begins running while Bertholdt grapples on to his shoulder with Eren. Ymir jumps on his back, and he carries them all away.
During their escape, Commander Erwin Smith leads a horde of Titans into Reiner to stop him and Ymir is forced to defend Historia. In the chaos they are separated from Reiner and Bertholdt, but they manage to kill a Titan together. As the battle intensifies, Ymir questions whether she is better off siding with the Warriors or the Scouts, but any way she looks at it, the chances inside the Walls are bleak.
She joins Historia, Sasha, and Conny in fending off the Titans around them when a newly freed Eren strikes a smiling Titan with his bare hand, triggering something that everyone with the power of the Titans can feel.
This causes all the mindless Titans present to drop what they are doing in favor of devouring the one that had attacked Eren. As Eren runs away he screams and sends the Titans after Reiner and Bertholdt. Ymir now realizes why they were after Eren, and that with his power the people of the Walls may have a future.
The Scouts prepare to retreat during the confusion and Ymir looks back to see Reiner and Bertholdt are about to be overwhelmed. She reaches out to Historia and caresses her hair before apologizing and turning to run back. Ymir arrives in time to stop a Titan from eating Bertholdt and joins them in their fight to survive.