To what extent does Chapter 1 of Dr. Jekyll and Mr. Hyde illustrate the view that Mr. Hyde's opposite is Mr. Utterson & not Dr. Jekyll?
The opening chapter of The Strange Case of Dr. Jekyll and Mr. Hyde establishes several structural and thematic points. One is that, since a story about Jekyll and Hyde opens with a lengthy discourse about Mr. Utterson, the structure of the story will pivot around Utterson's experience of the lives of Jekyll and Hyde.
The narration spoken to Utterson by his friend and cousin Mr. Enfield also establishes the structural opening for alternate narratorial voices in the text, such as the later epistolary (letter) chapters.
Thematic concerns are established by setting up Utterson and Enfield as two dark, somber, brooding individuals, the latter (second) fond of socializing and adventure, who have "something eminently human" beaconing (shining) from their eyes. Thematically, this sets them, especially Utterson, as the beacons to thrown light upon Mr. Hyde's character by showing that the thing deformed about Mr. Hyde is his lack of humanity, not his brooding spirit.
This is important to the themes because it resolves Jekyll's questions and contradicts Jekyll's theory about the valid possibility of separating one's good self from one's bad self, proving conclusively that the separated bad self is inhuman having neither humanity nor humane feelings.
The view that Utterson is the real opposite of Hyde, whereas Jekyll is Hyde's alter-ego (substitute persona), is fully supported by the opening chapter. The narrative told by Enfield sets Hyde and Utterson (and Utterson's sociable and adventuresome counterpart, Enfield) in oppositional juxtaposition. The character descriptions of Utterson and Enfield establish humanity as the measure by which to judge Utterson's opposite, Mr. Hyde.