The Attack Titan, like all the Nine Titans, was brought into the world after the death of Ymir Fritz. Grisha Jaeger knew of the Founding Titan's bloodline restriction from the future memories of his son, Eren. Eren Kruger received memories of Grisha Jaeger telling his son to protect Mikasa Ackermann and Armin Arlelt, although he did not know whose memories they were. These memories led those with the Attack Titan to move forward, fighting for freedom. While all of the Nine Titans could receive memories of past inheritors, the Attack Titan could also receive memories of inheritors yet to come.
3.3 The 57th Exterior Scouting Mission arc.
