We'll give Abe the first word on Speed: he calls Speed "a pestilent but well-meaning friend" (7.65). Which is a long way of saying Speed talks too much.
For instance, when Abe walks into the store where Speed is a clerk, Speed launches into a series of questions, not giving Abe any chance to get a word in (7.12). But against all odds, they become friends. And by "against all odds," we mean that Speed gives Abe a free place to stay. (It's surprising that they remain friends because it can be real hard to be friends with a roommate. Shmoop once roomed with our former best friends, the members of NKOTB, and… let's just say it didn't end well.)
That they're friends at all is even more impressive when you consider the fact that Speed's dad was one of those bad Southern slave-owners who used to sell slaves to vampires for food—just like Mary Todd's father (7.83). If you remember, Abe broke off his engagement with Mary because of her father's vampire dealings. But Speed's nothing like his old man, and as it turns out, his dad's cozy relationship with the toothy ones provides the perfect opportunity for some major slayage.
When he visits Speed's boyhood home, the two of them decide to con some vampires into coming to Speed's place to meet Abe's good friend, Mr. Ax. Abe gets all that bad blood between him and Mary out of his system by killing a ton of vamps, and when Abe gets back to Springfield after that trip, he's totally ready to marry Mary. Sure, Abe and Speed chat about Mary's past a whole bunch, but it's really the vampire-killing that puts Abe in the proper headspace for marriage.
So, through his friendship with Joshua Speed, Abe is able to work out his feelings about Mary Todd's father. Thanks to Speed, Abe feels better about marrying Mary, despite her family's sketchy past. After all, if Speed's an okay dude with a sketchy family, well then Mary could definitely be an okay woman with a sketchy family, too.
Speed is loyal to Abe—loyal enough that when Abe asks him to come out to kill Jefferson Davis, Speed comes. But he's not exactly cut out for the assassination business. When Abe, Lamon, and Speed are surrounded by evil Southern vampires, Abe and Lamon pretty much say, "Let's go out in a blaze of glory and kick some bloodthirsty butt!" By contrast, here's Speed's reaction:
"I'm wearing a watch," Speed told the executioners, his voice cracking. "It belonged to my grandfather, I—I ask only that someone see it back to my wife in Louisville." (10.129)
Now, we could make a joke about Speed's voice cracking, but we kind of love Speed here for thinking about the rest of his family—an heirloom from his granddad and getting it to his wife. If Speed isn't meant to be a vampire hunter, maybe it's because he cares too much for his living family, and isn't really willing to sacrifice that. Sure, it's not useful in a fight, but it's admirably sweet, if you ask Shmoop.