Nick

Pastor Nick (ニック司祭) was a member of a religion that worships the Walls and considers them to be holy.

Appearance
Pastor Nick was a very tall man at 6'4 and older looking with noticeable wrinkles on his forehead and around his eyes, which had dark circles over them. His hair was high up on his head and was parted to the left, with the sides and back being much shorter. He wore a black cassock with three golden colored bands near the top that had the symbols of the Walls on them.

Personality
Pastor Nick can be characterized by one word: extreme. Any attempt to interfere with the walls was a sin and considered the worst kind of offence in his mind. He never hesitated to make his opinion known and was extremely quick to condemn anyone who disagreed with him. This was proven true at the Trial of Eren Yeager, where he loudly called for his execution and publicly argued with a merchant who suggested sealing the walls. Any viewpoint besides his was considered horribly wrong.

However, Nick's personality was altered substantially once he was shown the extent of the war. Since he lived within Wall Sina, he never had to see the bloodshed, so the shock of seeing the destruction past the wall mortified him. He never again condemned anyone or made public his opinions as he became very quiet. He converted into a bleak and depressing man; only willing to speak when it was absolutely necessary. He did still retain the secretive aspect of his personality, and refused to reveal any information about the walls, even after seeing the horrors caused by the Titans. So great was his devotion, that even the threat of death could not sway him.

Nick did show signs of changing his viewpoint before he was murdered. While he didn't reveal any secrets about the walls, he did give the name of someone who could. His allegiance may have been changing since, as Levi theorized, he wouldn't reveal any information about the Survey Corps. to his torturer.

Background
Pastor Nick claimed that he lost his family as a result of addiction to alcohol.

As a religious leader, Pastor Nick used his influence to oppose any modification of the Walls, be it permanently sealing the gates or even mounting cannons on the top. Though he claimed it was blasphemy for humans to "maim" the Wall, the real reason was to reduce the risk of workers discovering the Titans within during renovations.

The Female Titan arc
Pastor Nick first appears when he advocates for the execution of Eren upon the revelation of his ability to transform into a Titan. He later appears preaching in a church about the sanctity of the Walls, when Eren punches the Female Titan into the building, seemingly killing everyone but Nick. Dread fills him as he realizes a Titan is inside the Wall, causing havoc across Stohess District.

Clash of the Titans arc
After Annie breaks a portion of Wall Sina and exposes a Titan within, Pastor Nick pleads that it be covered up immediately to prevent sunlight from awakening it. Realizing that Pastor Nick knew the truth about the Titans within the Walls the entire time, Hange Zoë threatens him in an attempt to get him to reveal more, but he refuses.

Hange brings Pastor Nick with her to Hermiha District so he can see, with his own eyes, the suffering of the families forced to evacuate because of the suspected breach of Wall Rose. Although daunted with the reality of war and the suffering it's caused on the people, he still does not betray the secrets of his organization but tells Hange about a girl, hiding under an assumed name, who would be able to speak about some of the secrets. Eren and the others realize Pastor Nick is speaking about Historia Reiss.

Pastor Nick is later seen with Levi and a few Military Policemen, watching as refugees flood into the town.

Following the traitors' failed attempt to kidnap Eren, and the refugees' return to Wall Rose, the priest is given a room in the same military barracks where Hange was recovering from her injuries. However, one morning he is discovered murdered, with several members of the Military Police Brigade's Capitol 1st Squad guarding the room. From a glimpse into his room, Hange realizes he had been tortured and then beaten to death. She immediately suspects the very officers carrying out the investigation of being the responsible party, and claims to be his old friend in order to inspect Djel Sanes's hands for evidence. Damage on his knuckles reveals his role in Pastor Nick's murder.

She relates this disturbing news to Levi and the surviving members of the 104th Trainees Squad, along with another disturbing conclusion. The evidence of prolonged torture prior to his death leads Hange and Levi to suspect a conspiracy within the interior, involving prominent members of the Military Police Brigade and other powerful figures. Levi states that the priest was likely tortured for information concerning the Survey Corps, but refused to betray them and was killed. Hange is deeply disturbed by his death, feeling responsible for overseeing his safety.

The Uprising arc
Later, Hange and Levi avenge his death by torturing Djel Sanes for information.