In Robert Louis Stevenson's novel, The Strange Case of Dr. Jekyll and Mr. Hyde, Dr. Jekyll tells Utterson at the very beginning that Jekyll and Hyde are tied closely together, but he will give no details.
'...there is one point I should like you to understand. I have really a very great interest in poor Hyde...I do sincerely take a great...interest in that young man; and if I am taken away, Utterson, I wish you to promise me that you will bear with him and get his rights for him.'
Utterson agrees, though having met Hyde, he has found him rude and unlikeable.
Some time later a horrendous crime is committed. Sir Danvers Carew—a gentleman—is murdered one night. A maid, looking out her window as is her custom, notices two men speaking. The first is the old gentleman and the second she recognizes as Hyde. All of a sudden, Hyde attacks the old man and bludgeons him to death with his cane. The matter is brought to Utterson's attention.
'And perhaps you can help us to the man.' And [the officer] briefly narrated what the maid had seen, and showed the broken stick.
Mr. Utterson had already quailed at the name of Hyde; but when the stick was laid before him, he could doubt no longer; broken and battered as it was, he recognized it for one that he had himself presented many years before to Henry Jekyll.
Though the reader cannot be sure what the connection is, the stick which Utterson had given as a gift to Jekyll has become a murder weapon in the hands of Hyde.
When Dr. Jekyll's letter is read at the end, we learn of the "creation" of Hyde.
I drank off the potion...There was something strange in my sensations, something indescribably new...I felt younger, lighter, happier in body...
I knew myself...to be more wicked...and the thought...braced and delighted me like wine...I was suddenly aware that I had lost in stature.
I determined...to venture in my new shape as far as to my bedroom...and coming to my room, I saw for the first time the appearance of Edward Hyde.
This segment describes the first time Jekyll takes the potion he has concocted. He describes what his body goes through during its metamorphosis, how different he is in nature (he feels wicked and reckless) and stature (he is shorter, but feels much younger).
Hence, although I had now two characters as well as two appearances, one was wholly evil, and the other was still the old Henry Jekyll...
And Hyde continues to appear:
Henry Jekyll stood at times aghast before the acts of Edward Hyde; but the situation was apart from ordinary laws...It was ...Hyde alone, that was guilty. Jekyll was no worse; he woke again to his good qualities seemingly unimpaired; he would even make haste, where it was possible, to undo the evil done by Hyde. And thus his conscience slumbered.
By holding Hyde responsible for his acts, Jekyll is able to maintain a quiet conscience. However, one night Jekyll goes to sleep as himself, but wakes as Hyde without taking the potion. From then on, Jekyll must continually take the brew to remain himself.
I...sicken and freeze at the mere thought of [Hyde], when I recall the abjection and passion of this attachment...I know how he fears my power to cut him off by suicide...
Jekyll hates Hyde, and Hyde knows his existence rests in Jekyll's hands. Jekyll decides that when he becomes Hyde again, he will not change back to Jekyll; he knows Hyde will commit suicide rather than be hanged for murder.