Nick

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

Background
Pastor Nick claims 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 appeals 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. Daunted with the reality of war and suffering it's caused on the people, he 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 was given a room in the same military barracks where Hange was recovering from her injuries. However, one morning he was discovered murdered, with several members of the Military Police Brigade's Capitol 1st Squad guarding the room. From a glimpse into his room, Hange realized he had been tortured and then beaten to death. She immediately suspected the very officers carrying out the investigation of being the responsible party, and claimed to be his old friend in order to inspect Djel Sanes's hands for evidence. Damage on his knuckles revealed 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.