Smiling Titan

The "Smiling Titan" is the unofficial name of the Titan that was responsible 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 Ackerman from the first assault of the Titan itself.

Appearance
The "Smiling Titan" was around 14-15 meters tall and like most regular Titans, was shown to lack intelligence and resembled a male human. Its main distinguishing feature was its characteristic, grinning smile, in which its lips are fixed wide open, exposing its teeth and gums within at all times.

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, as it's the only known Titan to kill its victims before devouring them.

Fall of Shiganshina arc
After the Colossal Titan breaches Shiganshina District in Wall Maria, 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, who is trapped under the debris of their house. Though Hannes initially comes to their aid and decides to face the Smiling Titan, he stands catatonic in fear of its frightening appearance. After he 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 proceeds 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 protecting Eren and Mikasa before he was devoured, the Titan reaches out for Eren, who punches its hand. This activates powers Eren was previously unaware of, and seemingly gains control over the surrounding Titans. The ones that are under his control proceed to kill the Smiling Titan by ripping it to shreds and devouring it.

The Uprising arc
As Squad Levi is preparing to battle Titan Rod Reiss, Eren sees three children looking at the top of the walls. He mentions those kids are just like he, Mikasa, and Armin when the Titans first attacked. Armin replies the only difference is that they, soldiers, are atop the wall to stop the Titan. This prompts Eren to think back to the day they first saw the Smiling Titan. He realizes how much Armin and Mikasa have grown, and begins punching himself for being a "useless brat."

Anime and Manga Differences
The scene of Kalura Yeager's death is depicted slightly different in the anime than in the manga. In the manga, the Smiling Titan squeezes her until her back breaks. Making her vomit her own blood, and twitch in its hand before it ate her. Whereas in the anime, it simply breaks her back by bending her backwards. Showing the Smiling Titan bite her with a loud crunch, with blood specks flying in the air.

Trivia

 * The Smiling Titan is the only known Titan shown to kill a victim before devouring them.