Smiling Titan

The "Smiling Titan" is the unofficial name of the Titan that was notable for killing Eren Yeager's mother, Kalura Yeager, during the invasion of Wall Maria. It was later devoured by other Titans directed by Eren, after killing Hannes, who saved Eren and Mikasa from the assault of the Titan itself.

Appearance
The "Smiling Titan" was 14-15 meters tall and it was shown to lack intelligence like other Titans in its class. Its main distinguishing feature was its characteristic grinning smile; in which its lips were fixed wide open, exposing its teeth and gums within at all times. In the anime, the sclera of its right eye was a luminescent yellow color, reminiscent of heterochromia.

The Smiling Titan had a thin emaciated frame with stocky proportions, in which its ribs were visibly apparent, and it possessed an unusually long neck. It had medium-length dark hair that only barely reached down to its head. Though it was not unlike many other Titans in terms of its instinct to seek out and eat humans, it's been shown to kill its victims before devouring them. It is the leader of the titan illuminati.

Fall of Shiganshina arc
After the Colossal Titan breaches the walls of the Shiganshina District, many Titans proceed inside and among them is the Smiling Titan. It embarks inside the town and is seen by Eren and Mikasa as they try to free Kalura Yeager under debris. Though Hannes initially comes to their aid and decides to face the Smiling Titan, he stands catatonic in fear of its frightening appearance. After Hannes realizes that he is too afraid to face it, he opts to instead take Eren and Mikasa to safety, leaving Eren's mother behind. Afterwards, Eren witnesses the Smiling Titan pull Kalura out of the debris, as it proceedes to violently incapacitate her and devour her shortly thereafter.

During Eren's days training to enlist into the military, it has a traumatizing impact on Eren. This is evident when he is approached by his fellow classmates about his impressions regarding normal Titans, in which Eren experiences a flashback of the Smiling Titan devouring his mother; associating normal Titans to the trauma caused by the Smiling Titan. He remains silent and is visibly shaken by the thought, until maintaining his composure in front of his classmates and shrugging it off as if he were unaffected by their questions.

Clash of the Titans arc
Five years later, after Eren had already joined the Survey Corps, the Smiling Titan appears again before Eren and Mikasa outside the Forest of Giant Trees during the military's pursuit of the Armored Titan. After killing Hannes, who was trying to save Eren and Mikasa before they were devoured, the Titan reachs out for Eren, who punches his hand. This activates powers he did not know he had, Eren seemingly gains control over the surrounding Titans. The ones that are under his control proceed to finally kill the Smiling Titan, by ripping it to shreds and devouring it.

Trivia

 * The scene of Kalura Yeager's death is depicted slightly different in the anime and manga. In the manga, the Smiling Titan squeezed her until her spine snaps, making her vomit blood and twitch in its hand before it ate her, whereas in the anime, it simply breaks her spine off-screen with both hands, showing the Smiling Titan bite downwards with blood spraying afterwards.
 * The Smiling Titan appeared in a Subaru Commercial advertising the live-action Attack on Titan movie.
 * The Smiling Titan is the only Titan shown to kill a victim before devouring them.