Who He Is
Demon in Woland's service, a thin, very pale man in dark glasses, named Abaddon. Woland shows him to Margarita in chapter 22, after saying that the retinue presented so far, Behemoth, Azazello, Korovyov, and Hella, is complete, and credits him with the work of a war that Margarita has just seen on the globe. He appears in chapters 22 and 23.
He did not have time to sin. Abaddon's work is impeccable.
Trajectory
In chapter 22, in Woland's bedroom, Margarita asks which side he is on. Woland answers and calls him softly: he emerges from behind the wall, the glasses frighten Margarita, and Woland does not let him take them off.
He is impartial and tends to feel compassion for both sides. As a consequence, the results for both sides are the same.
At the ball, three young men who resemble Abaddon stand beside Azazello, next to Margarita; at the end, Woland passes among him, Azazello, and others who look like him, and it is Abaddon who places himself before Baron Meigel, an instant before Azazello's shot.
Abaddon appeared before the baron and, for a second, took off his glasses.
Themes Embodied
The standard themes that run through him are War, Death, and Supernatural.