fiend
n. countable C2 Proficiency US //ˈfind// UK //fˈiːnd// fiend
n. someone who is very interested in or obsessed with a specific activity. It can also mean a person who is very cruel or wicked.
n. an individual with an intense, obsessive interest in a specific activity or substance; alternatively, a person of great wickedness or cruelty. Often used in compound forms to describe enthusiasts.
My brother is a total fitness fiend.
As a self-proclaimed coffee fiend, she refuses to start her morning without a double shot of espresso.
The antagonist was portrayed not as a mere criminal, but as a calculating fiend whose every action was designed to cause maximum psychological distress to his victims.
Usage
Commonly used in informal compounds like 'caffeine fiend' or 'exercise fiend' to denote obsession.