Final answer:
The narrator found Dupin's ability to figure out puzzling enigmas, his knowledge of the narrator's thoughts, and his seriousness about his gift fascinating.
Step-by-step explanation:
The narrator found several aspects of Dupin fascinating.
Firstly, the narrator was fascinated by Dupin's ability to figure out puzzling enigmas. Dupin had an extraordinary skill in solving complex mysteries.
Secondly, the narrator was amazed by Dupin's knowledge of what the narrator was thinking about. Dupin seemed to have the ability to read the narrator's mind.
Lastly, the narrator admired Dupin's seriousness about his gift. Dupin did not use his skills to deceive others or engage in charlatanerie.