What is Dr. Jekyll's background story?
He was born in 1818 and was quite wealthy. He says he was
"endowed besides with excellent parts, inclined by nature to industry, fond of the respect of the wise and good among my fellow-men, and thus, as might have been supposed, with every guarantee of an honorable and distinguished future" (Henry Jekyll's Full Statement of the Case).
In other words, he was born with every opportunity to be successful.
He goes on to say, in some foreshadowing of what he will later become, that he always had trouble reconciling the proper lifestyle he was expected to live (he was a gentleman in Victorian England after all) and his hidden desires. He decided, then, to live a double-life—one where he could be respectable and successful during the day, and another where he could give in to his less respectable needs at night. He says he did this secretly for a long time and he recognized he was literally of two different identities—both true identities.
"I saw that, of the two natures that contended in the Weld of my consciousness, even if I could rightly be said to be either, it was only because I was radically both" (Henry Jekyll's Full Statement of the Case).
He then set out to find a scientific way to separate the two.