Eliot Ness was a Prohibition agent and leader of a famous team of law enforcement agents nicknamed The Untouchables.
In the Timeless Universe
In the original timeline, Eliot Ness and the Untouchables are able to bring down Al Capone with the charge of tax evasion on March 13, 1931.
In the altered timeline, Garcia Flynn prevents Capone from being arrested due to tax evasion. Lucy Preston, Wyatt Logan, and Rufus Carlin, upon arriving to 1931 and seeing that Capone is not arrested turn to Ness in hopes of arresting Capone.
Ness leads Rufus, Wyatt, and Lucy to his abode to discuss taking down Capone, but he is found and killed by one of Capone's gangsters during the meeting. Flynn provided Capone with the knowledge of Ness' location. Because of this, Capone arranges a meeting with Flynn and Chicago Mayor William Hale Thompson. In the altered timeline, Ness dies on March 13, 1931.
Eliot Ness at Wikipedia