Wall Titan

The Wall Titans (幻の巨人) are unusual Titans that make up the Walls that protect humanity. They are first revealed when the Female Titan tried to climb Wall Sina to escape from Eren Yeager and the Survey Corps. The lack of sunlight inside the Walls renders them immobile. 21

Appearance
The Wall Titans are similar in appearance to the Colossal Titan in that both their surfaces are covered primarily in muscle tissue. Their exact height is not known, but seeing as they make up the Walls, it is implied that they are extremely tall.

Abilities
It is implied that Wall Titans have the ability of using their crystallizing powers to create massive Walls, though their exact motives for doing this are unknown.

The Female Titan arc
During the battle between Eren Yeager and Annie Leonhart in the Stohess District, Annie attempts to escape by climbing the Wall but is stopped by Mikasa. She damages part of the Wall however, and a huge chunk is torn off, revealing the face of a mysterious Titan.

Clash of the Titans arc
In the aftermath of the battle, the missing chunk off the Wall is immediately noticed by Mikasa. The Titan within the Wall follows Mikasa's gaze, and Pastor Nick calls for the immediate covering up of the Titan, as the sunlight eventually will enable its movement. After Nick ignores Hange Zoë's questions about it, he is nearly thrown off the Wall by her, but still refuses to reveal any details he might know because of the Wall Cult's forbidden nature to disclose secrets.