Stephen A. Douglas
Here's how crazy Illinois is in the 1800s: not only are Douglas and Abe political opponents, they're both competing for Mary Todd's hand in marriage. Awkward.
Douglas is almost the exact opposite of Abe. He's short and stocky and wealthy (compared to Abe) and also a Democrat (7.73)—where Abe is tall and thin and poor and a Whig (and later a Republican). Plus, where Abe is a staunch abolitionist, Douglas is more of a moderate.
But Lincoln wasn't always so anti-slavery. He was more like, "meh, slavery." He's fine with slavery in the slave states, but he doesn't want slavery in the northern states. That is, until 1854 when Douglas put forward the Kansas-Nebraska Act, which would allow slavery all over the country. Lincoln is so opposed to this that he gets back into politics (9.24).
In that sense, Abe's political career depends on Stephen A. Douglas. Sure, they fight and Douglas beats Abe at least once in that 1858 Senate race. But (a) Abe only gets back into politics because of Douglas and (b) Abe becomes more famous because of his debates with Douglas.
Both the debates of 1854 (9.27) and the super-famous Lincoln-Douglas debates (10.12) helped make Abe-the-character and Lincoln-the-man into the savvy, well-known politician he needed to be. So when Abe gives a speech against slavery and Douglas in Illinois in 1854, it makes him famous on a national level: "The speech was transcribed and reprinted across the North. The name Abraham Lincoln began to take on national significance among the opponents of slavery" (9.33).
All in all, ol' Dougie is the anti-Abe, except for one thing (well, two if you count that both of them wanted to marry Mary Todd. Zing.): both Abe Lincoln and Stephen A. Douglas are against the idea of secession. And when Douglas is told by Abe that the current enemies are vampires, Douglas is shocked and dedicates himself to fighting for the Union.
That tells us that he's a bit of a doofus. Really, how could you not know that your Southern buddies were vamps this whole time? But it also tells us that he's a patriot, which is a-okay in Abe's book.
Unfortunately, Douglas dies under mysterious circumstances, with tiny holes in his neck and no blood. We suspect a mummy was to blame.