Ezra Jennings is one of the weirder characters in The Moonstone, which is saying a lot. He's important to the plot because he is the one who figures out that Franklin Blake must have been drugged with opium the night that he took the Moonstone. But couldn't he have just been a regular old doctor's assistant? Why is he also a social outcast? Why is it important that he have this scandal attached to him? And what on earth is up with his hair?
Some critics like to argue that Ezra Jennings is a kind of double of Wilkie Collins himself. Like Jennings, Wilkie Collins was an opium addict. Like Jennings, Collins first started taking opium for medical reasons, to treat chronic pain. Like Jennings, Collins was the subject of some scandalous rumors. Of course, it's easy to trace parallels between an author and his character, but it's a lot harder to prove what the author's intentions were, so usually it is best not to try.
Let's stick with what we do know about Jennings: Franklin Blake describes him as having "dreamy eyes" and a "gipsy complexion," and says that "there was the mixture of some foreign race in his English blood" (188.8.131.52). Maybe this explains why Franklin Blake and Ezra Jennings get along so well. Franklin Blake is English on both sides, but he has spent so much time living outside of England that, as Betteredge is fond of remarking, he has "foreign sides" to him. And Ezra Jennings is half English, and half something else (we're never told what nationality or race his mother was). Jennings's hint of foreignness is part of what makes most English people, like Gabriel Betteredge, distrust him. At the same time, it might be what Franklin Blake likes about him so much.