Bertolt Hoover was an Eldian and one of Marley's Warriors. Brainwashed by propaganda as a child, he was led to believe that his actions of terrorism he would commit would be honorable and just. He possessed the ability to transform into a Titan known as the Colossus Titan. In the year 845, he, Reiner Braun, and Annie Leonhart breached Wall Maria in an attempt to lure out and acquire the Founding Titan for Marley. After that, he joined the military and graduated the 104th Training Corps, ranked 3rd, and later joined the Survey Corps, until his identity as the Colossus Titan was revealed. He kept this power until he was killed by Armin Arlert. He was described by others to be a weak-willed and quiet person.
Bertolt was a rather tall, slender young man. He was the tallest of all currently known holders of the power of the Titans. He had short, dark hair, brown eyes, thin eyebrows and an elongated face. He was seen mainly in his Trainee uniform, and after he entered the Survey Corps, he used their uniform. After his encounter with the Beast Titan, his hair appeared shorter and wore a trench coat. During the Warriors' battle in Shiganshina against the Survey Corps, Bertolt wore a set of vertical maneuvering equipment along with a collared shirt and dark pants.
Bertolt was a quiet young man who identified himself as someone with a weak will that follows popular opinion. Likewise, his instructor noted him to be a person with great potential, but an utter lack of initiative. He was further described by Eren Yeager as being taciturn, and Bertolt tended to remain in the background unless necessary. By his own admission, he was somewhat of a coward and was noted for his low self-esteem.
Appearance: Bertolt was a rather tall, slender young man. He was the tallest of all currently known holders of the power of the Titans.
Personality: Bertolt was a quiet young man who identified himself as someone with a weak will that follows popular opinion. Likewise, his instructor noted him to be a person with great potential, but an utter lack of initiative.
History: Bertolt was an Eldian born in the continent of Marley. At some point in his childhood, he joined the Marleyan military as a Warrior candidate.
Bertolt Hoover in the story: Bertolt transforms into his Colossus Titan form at Shiganshina District, terrorizing its population and kicking a hole in the district's gate. The attack leads to falling debris scattering across the town, crushing houses in the process. Immediately after his attack, Bertolt reverts to his human self, while Titans enter the city from outside the walls, resulting in the population being set on by them.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.