Short answers, pulled from the story.
Delaware, Maryland, Kentucky, and Missouri formed the Border South group that stayed within the Union while their southern neighbors seceded. A fifth jurisdiction emerged as West Virginia from western Virginia counties by 1863.
President Abraham Lincoln stated that losing Kentucky would be nearly equivalent to losing the entire game because its geographic position made holding Missouri or Maryland impossible without it. The state's location meant Washington D.C. could be surrounded if neighboring Maryland joined the Confederacy.
The Kentucky legislature passed two resolutions of neutrality on the 20th of May 1861 asking both sides to keep out of the state. Confederate General Leonidas Polk occupied Columbus, Kentucky in summer 1861 which broke this neutrality.
About 170,000 border state men fought in the Union Army while 86,000 served in the Confederate Army during the conflict. Approximately 80,000 to 125,000 Kentuckians served as Union soldiers and an estimated 35,000 to 40,000 served as Confederate soldiers.
A statewide convention voted for secession on the 17th of April 1861 after Fort Sumter's attack and Lincoln's call to arms. The vote favored secession 132,201 to 37,451 but western counties later formed West Virginia with a majority against secession.
Maryland abolished slavery in 1864 while Missouri and Tennessee did so in January 1865 and West Virginia followed suit in February 1865. Delaware and Kentucky saw substantial reduction in slavery but did not see abolition until December 1865 when the Thirteenth Amendment ratified.