Anduriel

Anduriel is a fallen angel hosted by Order of the Blackened Denarius member, Nicodemus Archleone. He first appears in Death Masks.

Description
This relationship appears to be an equal partnership which gives Anduriel more power because he can utilize his host's free will to a greater extent.

He appears to manifest as Nicodemus' animated and corporeal shadow, which can suppress the magic power of human practitioners in a similar manner to thorn manacles, as well as directly assist Nicodemus in combat.

He was a captain of Lucifer's after the Fall. He leads the thirty Fallen who inhabit the coins.

Death Masks
In Death Masks, Harry meets Nicodemus and Anduriel for the first time. He notes the way Anduriel moves in Nicodemus's shadow, and sees some of what he can do.

Small Favor
In Small Favor, Harry meets Nicodemus at the Shedd Aquarium in the entry hall, and is disturbed when he sees Nicodemus standing still while his shadow followed a shark. When they were alone, Anduriel began gliding around the walls, circling Dresden.

After Dresden reveals that at least one Fallen Angel is assisting Nemesis Nicodemus is shocked and throws Dresden against the wall, demanding to know if he's lying. Anduriel emerges partially, opening "faintly glowing" green eyes over Nicodemus's own, and when Dresden doesn't fight back Anduriel speaks for the first time, commenting that Dresden at last showed some survival instinct.

Skin Game
In Skin Game, Harry Dresden comments that he has no idea what Anduriel can do, because Nicodemus has never had to fall back on him. Kringle explains that Anduriel, while deadly in battle, is not a fighter by nature; instead, he functions as a spymaster for the Denarians, able to see and hear anything within range of a living being's shadow. He has to focus to hear or see what a particular person is done, and can come almost anywhere; the known exceptions being Demonreach, the Carpenter home, or in Mab's presence.

Word of Butcher
The author has stated that Nicodemus has no other alternate form other than his animated shadow.

"Does Nicodemus have any alternate forms other than his shadow?

No, He just has his shadow do things for him. You go relying on an alternate form to get things done, that still puts you in personal danger and Nicodemus is more practical than that. He'd rather stand over here and let something else kill and get the work done. Unless it's something cool like a Knight of the Cross, in that case he's still got something to prove."