Also known as Lewis Powell. From wikipedia:

On April 13, John Wilkes Booth, George Atzerodt and David Herold all met at Powell's room at the Kirkwood Hotel, where Booth assigned roles. On April 14, Powell was to go to the home of Secretary of State William H. Seward and kill him, accompanied by Herold. Atzerodt was to assassinate Vice President Andrew Johnson; he would fail because he lost his nerve and got drunk.[3] Booth was to assassinate President Abraham Lincoln, a task he completed.

Powell attacking Frederick Seward after attempting to shoot him. On April 14, 1865, Powell was escorted to the Seward residence by David Herold. Powell gained entry into the house by claiming that he had medicine for Seward from Dr. Verdi. (Earlier in the month, on April 5, 1865, Seward had been injured in a carriage accident, and suffered a concussion, a broken jaw, a broken right arm, and many serious bruises. He was at home convalescing.) Powell then attempted to kill William Seward by breaking into his bedroom and stabbing him repeatedly. A jaw splint worn by Seward helped to save his life by deflecting the knife away from his jugular vein. Powell also injured Seward's two eldest sons (Augustus and Frederick), his assigned military nurse, Sergeant George F. Robinson, and messenger Emerick Hansell, who arrived right as Powell was escaping. After the attempt on Seward's life, Powell threw his bloody knife up into the gutter of the Seward house and fled on horseback.[2] He discarded his light-colored coat in a Washington suburb cemetery where he hid.[2] At some point, the horse, purchased by John Wilkes Booth in December 1864, that Powell was riding either threw him or he fell off.[2] The horse was later found near the Lincoln Branch Barracks, close to the Capitol.[2] After hiding out in a tree for three days, Powell went to Mary Surratt's boardinghouse only to arrive at the same time that she was being arrested for her part in the assassination.[2] Although it was night time, when asked why he was there, carrying a pickaxe, Powell claimed that he had been hired to dig a gutter.[2] Surratt denied knowing who he was, despite his having visited and stayed at the boardinghouse on several occasions.[2] Powell was arrested and taken into police custody. After Seward family servant William Bell picked him out from a police lineup, Powell was taken to the Washington Navy Yard, where he was confined aboard the monitor gunboat USS Saugus.[2] Powell and the other surviving conspirators were later transferred to the Old Capitol Prison.

He was eventually executed.

Those clothes actually look pretty modern.