An enthusiast — someone deeply interested in and often very knowledgeable about a particular field or hobby.
彼は鉄道マニアで、週末はよく写真を撮りに行く。
He's a railway enthusiast and often goes out on weekends to take photos.
マニアの間ではその限定版が話題になっている。
That limited edition is a hot topic among enthusiasts.
料理マニアとして新しいレシピを試すのが好きだ。
As a cooking enthusiast, I like trying new recipes.
Mania/obsession — intense enthusiasm or obsession itself; often used to describe maniacal interest or something 'maniacal' in nature.
彼の映画へのマニアぶりは時に周囲を困らせる。
His mania for movies sometimes troubles those around him.
新製品の仕様を細かくチェックするのはマニアの性だ。
Checking new product specifications in detail is the nature of an enthusiast.