A Union Army colonel of the American Civil War and ancestor of William Riker, whose life was saved on the battlefield by a member of the Q Continuum, as revealed in Death Wish.
Colonel Thaddius Riker, nicknamed Old Iron Boots, was a Union officer in the American Civil War and a forebear of Starfleet's William T. Riker. He is established in the Voyager episode Death Wish.
During the asylum hearing of the Q later known as Quinn, Q summons William Riker to Voyager as a witness and shows him a photograph of Thaddius, who commanded a New York infantry regiment under General Sherman. Riker knows the family story that his ancestor was wounded at Pine Mountain in 1864 and carried to safety by an unknown soldier.
Q reveals that the rescuer was Quinn. Without him Thaddius would have died, there would have been no William Riker, and the Federation's defence against the Borg would have gone very differently. Jonathan Frakes appears as both Rikers, the colonel being seen in the photograph.