Mr. Hyde is Dr. Jekyll’s evil alter ego who indulges in various undisclosed vices. He’s smaller, younger, more energetic, and just basically a malevolent, villainous guy. He’s frequently compared to a monkey or an ape, suggesting a certain inhumanity or bestiality. Although no one can really pinpoint a particular deformity, they all agree that he has one – one that makes him a twisted, dark man who manages to inspire fear, disgust, and loathing even from afar. He is also described repeatedly as "timid yet bold."
He indulges in many undisclosed pleasures, but the main characteristic we see is that of violence. Mr. Hyde, to put it mildly, likes to beat people up. Based on the crimes we see, his predilection for violence isn’t like a gang member’s, because he doesn’t hold any affiliations or have any conception of honor and respect. Nor is he really like a schoolyard bully, because he’s not particularly bigger or stronger than the people he beats up – not to mention that he doesn’t do it in front of crowds or to make himself look good. No, he simply likes beating people up. He feels pleasure when he engages in violence.
Mr. Hyde’s name is also significant, as he is both a hidden man and a persona that Dr. Jekyll hides behind. Mr. Utterson notes the significance of the name in Chapter Two: "‘If he be Mr. Hyde,’ he had thought, ‘I shall be Mr. Seek.’"
Perhaps the most interesting aspect of Mr. Hyde to discuss is his death. We aren’t really sure at the end how Mr. Hyde died. Well, suicide, you say, but who committed the suicide? We know that Hyde is afraid of death (and probably the Hell which, according to Victorian England, awaits him), and that the threat of suicide is Jekyll’s one weapon against him. So one would think Jekyll somehow killed himself and Hyde right after writing out his tale. That’s the easy explanation.
Jekyll did say that Hyde was taking over, growing stronger and whatnot. If the last thing Jekyll was able to pull off was writing his last "document," then it sounds like Hyde took over before the death occurred. So did Hyde kill himself? And does that mean he killed Jekyll? You could say that Jekyll was already dead by that time, that Hyde had taken over completely. After all, it is Hyde’s dead body that we see, not Jekyll’s.
Lastly, you could say that it’s irrelevant to ask who killed himself, or who killed whom, because at the end of the day, Hyde and Jekyll aren’t separate entities. They share the same body, and the same memory. You could even go so far as to say that Jekyll’s attempted division failed; man can’t be separated simply into two halves.