devotee
n.a person who is very interested in a specific activity, hobby, or belief. You use this to describe someone who spends a lot of time and energy on one thing.
n.a person who is deeply committed to a particular activity, belief, or person. Often used to describe a follower of a specific religion or a passionate enthusiast of a hobby.
“He is a devoted devotee of classical music.”
“As a devoted devotee of the sport, she attends every local match and follows the international league closely.”
“The philosopher was a devoted devotee of the ancient texts, spending decades in quiet study to understand the nuances of the original language.”
“NPR's Linda Holmes is a daily puzzle devotee.”
“Devotees carry these idols through the streets before immersing them in the Arabian Sea as part of the annual 10-day Ganesh Chaturthi festival.”
From devote + -ee. The narrowed meaning in sense 3 arises from the sound similarity with amputee.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.