Attack Titan (Anime)

The Mysterious Titan (不思議の巨人) is the Titan form of Eren Jaeger. His Titan first appeared during the struggle for Trost and has become a symbol of humanity's newfound hope ever since.

Appearance
The mysterious Titan is a class with shaggy black hair, pointed ears, and an extremely muscular physique. The mouth of the Titan possesses no cheeks or lips, exposing a jagged row of many teeth.

The Struggle for Trost arc
After a Titan's arm bursts from the Bearded Titan's throat, out steps the Mysterious Titan from the Titan's back. He then proceeded to wreak havoc on all the Titans he could get his hands on. In the process, he unknowingly saves the mourning Mikasa’s life and is baited by his classmates to help fend off a procession of Titans from a storage headquarters for the compressed gas used to power their ODM gear.

As the Titans seem to overwhelm him and Mikasa’s group is about to intervene, the Titan that killed Thomas appears and the Mysterious Titan becomes enraged, throwing off other atackers to kill this newcomer. As soon as the massacre is over, the Mysterious Titan falls to the ground and his human form emerges from the back of his Titan form’s neck (with both lost limbs back) to the shock of all the onlookers. He is retrieved by an overwhelmed Mikasa.

A plan is made for the Mysterious Titan to use his Titan form to move a huge boulder they had planned to use to reseal Trost's gate to prevent Titans from further entering, but were unable to move. the Mysterious Titan is assigned with a team including Mikasa to defend him while he carries the boulder. Upon his transformation to execute the plan, though, he attacks Mikasa, who tries to communicate with him to snap out of it, unsuccessfully. the Mysterious Titan ends up wounding himself and his body fails to regenerate, the mission is declared a failure. Despite this, the leader of the group, with some non-verbal convincing by Mikasa, decides they must stay and guard the Mysterious Titan.

Armin arrives on the scene and convinces Mikasa to go help fend off the Titans while he tries to get through to the Mysterious Titan. Eventually he is able to do so, by reminding Eren that they wanted to see the world together, causing Eren to remember how he wanted to join the Scout Regiment, and destroy all the Titans. The Mysterious Titan moves the stone and is able to plug the hole in the Wall at the sacrifice of the rest of the team members (with the exception of Rico, Armin, and Mikasa). They are saved from the desperate situation after the Mysterious Titan’s transformation back to human by Levi.

Assault on Stohess arc
After transforming into a Titan, the Mysterious Titan assists in capturing the Female Titan, as the two brawl out in Stohess, the Mysterious Titan nearly gets defeated until it grows into a state of fury. The Mysterious Titan then turns black and starts to burn as it goes to attack the Female Titan, it succeeds in capturing her by ripping her apart and biting the nape open.

Clash of the Titans arc
After Reiner Braun and Bertholdt Hoover reveal themselves as the Armored and Colossal Titans, they transform into Titan form. The Armored Titan grabs Eren but he transforms into the Mysterious Titan to fight the Armored Titan.