I think that one common trait that the three characters share with one another is that there is a great deal of confusion or challenge in seeking to define them. Each of them feature their own set of paradigms of obscurity which makes it difficult to fully grasp who they are or an absolute feel of confidence regarding their identity. Along these lines, though, the questioning that the characters undergo is reflective of the questioning that we, as individuals, also undergo in seeking to understand how these characters' imprint is placed on themselves. Interestingly enough, how we understand the characters is a reflection of how we can understand our own sense of self.
Hesse's construction of Siddhartha is one in which identity starts off vastly different from what it is by the end of the narrative. He undergoes massive change and a sense of questioning permeates his own identity. Siddhartha seeks enlightenment and a sense of what is permanent and lasting in a world that is far from it. In much the same way, Cyrano constantly struggles with his own sense of identity. The concealment of who he is eats away at his self- esteem and sense of worth. His struggle to overcome this or at least place it in the correct sense of context is a part of his own identity. For Jekyll/ Hyde, the idea of duality within human identity is explored and established. "Those provinces of good and ill which divide and compound man's dual nature" become the critical demarcation of human identity. In each case, the idea that one's identity is something that must be examined and reexamined, without simplistic answers is something that links each character to one another. At the same time, this struggle for identity is something that the characters experience and we, as readers, seek to make sense of in our own existence.