Asmodai
Oh shit......
Asmodai is the Master Interrogator-Chaplain of the Dark Angels. He is the oldest and most experienced of the Interrogator-Chaplains (hence his rank), and also the most obsessed with hunting down the Fallen Angels and making them repent.
His obsession has grown and bled into the other aspects of his life as a Space Marine, and he's basically become the "fun police" of the 41st millennium. When a bunch of recruits didn't answer his questions as enthusiastically as he liked, he had them all killed because he thought their gene-seed might be contaminated. When he heard someone from the Seventh Company laughing while inside the Rock, he had the entire Company sworn to silence except for hymns and communicating in battle. His preferred tool of interrogation is a torture weapon called "the Blades of Reason".
Azrael has had to rein him in or cover up for some of his more excessive actions, but he keeps Asmodai around because, say what you will about his sense of humor (or lack of it), he delivers -- if there's a Fallen Angel (or even someone who might know something about a Fallen Angel) out there, he will track it down.