Dr. Van Helsing
Considering how important Van Helsing is to the novel, he doesn't get very much back-story. We know that he's a professor, that he's Dutch, and that he's an expert in pretty much everything. Besides knowing all about medicine (both modern and ancient), superstitions, and religion, he also lets drop at one point that he has a law degree, too!
But Van Helsing isn't just a walking brain: he has enough personal eccentricities to make him an interesting character. His accent, obviously, makes him stand out among the other characters – he's the only member of the Crew of Light (the group that hunts down Dracula) who isn't a native English speaker. The only other non-native speaker in the novel is Dracula himself. Besides the accent, Van Helsing has a number of other quirks: for example, he laughs hysterically when things seem pretty grim to everyone else (13.79). He also lets a few interesting facts drop about himself – we know that Dr. Seward saved his life once (9.26), and we know that Van Helsing is married, but that his wife is insane and therefore living in a mental institution (13.85).
In short, Van Helsing's eccentricities make him stand out as one of the most interesting, memorable characters of the novel. In fact, many readers tend to forget about the other members of the Crew of Light altogether, and just remember Van Helsing as the vampire fighter. Just think about the 2004 movie Van Helsing starring Hugh Jackman – no, that movie doesn't look much like Bram Stoker's Dracula, and Hugh Jackman is certainly younger and better looking than Stoker's Van Helsing, but you get the picture. Van Helsing, as a character, stands out enough that he can be taken out of the context of the original story.