Hermann begins his narrative exposition with a wide array of dithering and seemingly extraneous background information, coming in weird clips and decisive backsliding (as to almost appear overwrought to the point of being premeditated / too carefully crafted to fit the image he's creating for himself), expressing thoughts in an oddly whimsical way at times (e.g. "Tum-tee-tum. And once more -- TUM! No, I have not gone mad. I am merely producing gleeful sounds."), and all of which supposedly written as such for the simple fact of his culling it totally and completely from memory. Anything he knows is speculation he goes to great pains to make it abundantly clear that that is just what it is, speculative -- thus leaving everything to be doubted more so than usual when offered from the point of view of an unreliable first person narrator. His emphatic conviction makes him an untrustworthy individual, indeed. It's also a very nice bit of story-craft by Nabokov in those early, tone-setting few chapters.
The substance of the story, meanwhile, begins in the midst of everything else (his relationship with his wife, her cousin, his parents and so forth), when he describes coming upon a complete stranger named Felix, a man on the 180-degree opposite end of the socio-economic spectrum, a drifter and a panhandler whom Hermann believes to be his exact, perfect double -- uncannily resembling him in near every way, including countenance and stature. So is the seed of Hermann's obsession planted. Whatever logical end or imperative Hermann contrives to justify his plans, it's rendered moot and arbitrary when juxtaposed against his interest in the world perceiving his double as he does, i.e. possessing a flawlessly similar likeness. It takes a special kind of lunacy to aspire to this sort of social affirmation, going to the length of declaring ". . . I longed, to the point of pain, for that masterpiece of mine . . . to be appreciated by men . . ." And as the plot becomes more involved you sometimes wonder if Hermann thought Felix was handsome. I myself couldn't shake the joke of any person's flattering him or herself with the belief that they look like a certain celebrity or other-type beautiful person, even if the resemblance is tenuous at best.
Ultimately, it's beside the point whether there is something in Felix's physical appearance that Hermann admires. The real issue at stake is what possesses Hermann to prove to the world that their likeness is exact, to go to the extreme he hints at throughout the first many chapters of his tale. That's difficult even to theorize about. But it certainly invites your hypotheses. I'm always interested in the process by which humans either reject or accept someone into their fold, what sorts of traits one needs to possess to be accepted by one or an entire group at large. So accepted was Felix by Hermann, you might say, that the latter wished to adopt the life of the former, which is no small thing when you consider the difference of each man's social standing. There were facts about Felix that Hermann at least implicitly admires, even if more overtly he considers him a buffoon and a moron. His anonymity, in itself, seems to be a virtue. Felix travels all over with little more than a few personal items to show for it, to tangibly testify to his ever existing. Hermann wants a live evanescence, to become a shadow. Felix's being listed on his passport by occupation as a "musician" has a certain appeal to Hermann, as well. He might then be able to rid himself of his dislike of mirrors, which I'm sure there's some serious psycho-analysis to be done with that aspect of his character, in and of itself.
But Hermann is undone in his plans. I won't get into precisely how, but things don't go quite the way he'd hoped, or contrived. Instead, he is forced to deal with the little that is made of his "masterpiece" and the shame that no one but himself seems to think it is a striking resemblance.
But where Hermann especially fails is with his assumption that he is the smartest man in the room, when in truth many of the others whom he assumed to be fools, reprobates and / or oblivious were in fact on to him more or less the whole time (this seems especially true in the case of Ardalion, his wife's cousin). That and, basically, there no such thing as an exact double, so don't get any murderous ideas.