Bespectacled and balding, he has dark hair and beard, streaked with white, and dresses very conservatively.
Peabody compiled the book Die Lied der Erlking on Erlking lore, sometime "early last century" (presumably the 20th). He either does not speak German or is very bad at it, as he published his book Die Lied der Erlking with a grammatically incorrect title. He called German an "untidy language".
In the series
He's present at the Council's Conclave.
In "War Cry", he performs the spell required to bind the shoggoth to Catherine Taylor, over her objection. He later reports about its destruction, defining it as a temporary setback; a new weapon will be found in its stead.
He's present at Molly Carpenter's hearing.
In Turn Coat, when Donald Morgan was accused of the murder of senior council member Aleron LaFortier Dresden hid the fugitive Morgan from the Wardens. Harry Dresden arrives to White Council headquarters to secretly get more information. While there, he notices Peabody flitting between Senior Council members getting their signatures for this or that like an annoying gnat. He even tried to get Harry's signature and became quite incensed when Harry refused.
At Morgan's trial, Dresden accuses Peabody of being the real murderer who framed Morgan, manipulated the younger Wardens and subtly influenced the minds of the Senior Council. Ebenezar McCoy and Carlos Ramirez searched his rooms and found that the inks he used to gain signatures contained alchemical substances that allowed Peabody to manipulate persons to do his bidding.
Ultimately, in order to escape, Peabody smashes an ink bottle that freed a Mistfiend laced in Mordite and using pre-implanted trigger phrase "the end is nigh" to immobilize the young Wardens as he fled. Harry gives chase though the Hidden Halls of Edinburgh. He freezes Wardens at all of the checkpoints repeating the phrase. Peabody opened a way to the Nevernever, then he overpowered Harry Dresden and was about to kill him when Donald Morgan shot Peabody twice in the head with Anastasia Luccio's gun, killing him.
According to Rashid, Harry Dresden was virtually the only young warden who Peabody didn't get a chance to exploit with his mental manipulations. Harry might notice something wrong since the other young wizards like to associate with him. Peabody likely pushed Anastasia Luccio into Harry's arms to keep tabs on him and to possibly take him out if necessary.