The Attack Titan was one of the Nine Titans that could see through the memories of both its past and its future holders, and has fought for freedom throughout the generations.
Abilities: While all of the Nine Titans could receive memories of past inheritors, the Attack Titan could also receive memories of inheritors yet to come. Zeke Jaeger described it as "the power to transcend time", speculating that Eren sent specific memories to Grisha to suit him. These memories led those with the Attack Titan to move forward, fighting for freedom.
History: The Attack Titan, like all the Nine Titans, was brought into the world after the death of Ymir Fritz. Though this Titan was presumably caught up in the conflict of houses between the eight Titans of the Eldian Empire subservient to the Founding Titan, it is said that the Attack Titan always fought for freedom of no matter the day and age. During the Great Titan War as Marley gradually gained control over seven of the Nine Titans, the Attack Titan evidently never fell into Marley's hands and remained on the mainland while the Founding Titan remained overseas on Paradis Island where the 145th Fritz King, Karl Fritz, relocated the capital.
Story: After holding the Attack Titan for thirteen years, Wall Maria comes under attack by the Warriors of Marley, and Grisha decides the time has come to negotiate with the Founding Titan or take it by force. Arriving at the Underground Chapel, Grisha pleads with Frieda Reiss for the sake of Eldia and the Walls, but his cries are neglected. Although insistent about taking the Founding Titan by force, Grisha is unable to until he is pressured to by the future holder of the Attack Titan: his son Eren Jaeger.
Trivia: The Attack Titan shares the same Japanese name with Attack on Titan: 進撃の巨人. Eren's Titan form is modeled after the body of mixed martial artist Yūshin Okami.
Source: Attack Titan (Anime), available under CC BY-SA 3.0.