At the very beginning of The Strange Case of Dr. Jekyll and Mr. Hyde, Mr. Utterson is identified as a lawyer, whose appearance denoted a
rugged countenance that was never lighted by a smile
This entails that he is a person who adopts a serious, perhaps even rough demeanor that never changes.
He is also described as
cold, scanty, and embarrassed in discourse
which means that he does not show a lot of emotion in his overall demeanor, is quite demure, and not overly expressive. In short, he is the epitome of the Victorian "gentleman." He embodies the behaviors expected of a man who is educated, respectful, self-possessed, virtuous, and even religious.
He is definitely not "fun" to be around, since he is described as:
lean, long, dusty, dreary
Still, as a true gentleman, he is also a good friend that can be relied upon. This is evident when he is described as someone who still can be
The rest of the description states that Mr. Utterson has the capacity to emit 'something eminently human [..] from his eye' when he enjoys his wine. This means that Utterson can let his guard down a bit after drinking wine, like other people do.
Mr. Utterson is also described as someone "austere," that is, someone who would forbid himself from certain enjoyments. In order to restrain his taste for good "vintages" he would make himself drink gin, which tastes drastically bitter and is more medicinal- tasting, in comparison. That is pretty harsh!
Perhaps the strangest aspect of Mr. Utterson's austerity is that he loves the theater but never goes there, and has not gone to the theater in twenty years.
The most positive aspect of Mr. Utterson's behavior is his "approved tolerance for others." This means that he has no prejudice against anyone and is capable of befriending or socializing with people of all walks of life.
This is a big deal, actually. In Victorian times, optics were everything, and acquaintances reflected one's own social standing. A serious, virtuous, dry, and dreary lawyer willing to associate with strange and shady characters denotes someone who, despite his seriousness, is willing to take bold and risky steps that may not be socially accepted. This is an interesting dimension of his personality and proves that nobody is entirely "black and white" when it comes to personal choices. There is a "gray" area in everyone.