Nile Dok is a minor antagonist and chief to the Military Police Brigade.
Nile Dok was member of the same Trainee Squad as Erwin Smith, and the two were good friends. He even considered joining the Survey Corps, but after he he started meeting a woman named Marie rathger frequently, he abondoned his friend Erwin to join the Military Police to be with her.
The Female Titan Arc
We first see Nile overseeing the trial along side Erwin Smith, Dot Pixis, and Darius Zackley. He avocates for killing Eren, due to a possible uprising that could occur if left unchecked. Though he does recognize that Eren saved Trost, he also realizes that his powers could threaten the balance of power in human society. Though Nile loses the trial, he continues his duties as head of the Military Police Brigade.