This one's super-easy: Siddhartha is the name of the novel’s protagonist.
Oh, but here's a tip—keep in mind that our main character Siddhartha is not intended to be Siddhartha Gautama, popularly known as "Buddha" (as in Buddhism). The Siddhartha who lends his name to the novel’s title is a just man living in ancient India at the same time as Buddha.