Enthusiast (noun)

1. A person who is very interested and excited about a particular subject or activity.
2. A person who is passionate about something.

Origin:

From the greek enthousiastes, from enthousiazein (to have a god within, to be inspired by a god).

Examples:

1. She is an enthusiast of classical music.
2. He is an enthusiast of old cars.
3. The enthusiast spent all his free time working on his hobby.
4. The enthusiast was knowledgeable about the subject and was always happy to share his knowledge.
5. The enthusiast was passionate about his cause and worked tirelessly to promote it.