There were many "Southern loyalists," usually called "Southern Unionists," who attempted to assist the Union in a variety of ways during the war. In his Personal Memoirs of U. S. Grant (New York: Charles L. Webster & Co., 1885), General Grant wrote "We had many regiments of brave and loyal men who volunteered under great difficulty from the twelve million belonging to the South (Vol. 2, p. 636)." Standing out from the great number of Southern Unionist who enlisted in the Union Army / Navy were individuals such as Colonel Fielding Hurst of the 6th Regiment Tennessee Volunteer Cavalry, Admiral David Farragut, Rear Admiral Samuel Phillips Lee, Rear Admiral John Ancrum Winslow, Major General George Henry Thomas, Brevet Major General Robert Anderson, Brevet Major General Montgomery Meigs, Captain Samuel Means, 2nd Lt. Joseph G. Sanders (Capt. in CSA until 1864), and Brigadier General William R. Terrill, whose brother James was a Confederate officer posthumously promoted to the rank of Brigadier General.

One must also remember the civilian Southern Unionists, who did play a significant role in the shaping of American history both during and after the Civil War. Lincoln's second vice president, Andrew Johnson, was a Democrat from Raleigh, North Carolina. The state of West Virginia separated from Virginia after 30 of 49 delegates from the northwestern corner of the latter state voted against seceding from the Union at the Secession Convention on April 17, 1861, and were massively outvoted by the rest of the state delegates. Several conventions at Wheeling followed, culminating in the creation of the state and its admission to the Union on June 20, 1863. It was the only instance of a Union state being created out of part of a Confederate state.

If you are interested in reading on the subject, might I recommend James Alex Baggett's The Scalawags (Baton Rouge: Louisiana State University Press, 2003) and Richard Nelson Current's Lincoln's Loyalists: Union Soldiers from the Confederacy (Boston: Northeastern University Press, 1992), among many other fine books. I hope this has helped.