noun
A person who supports a person, idea, movement, or organization. Neutral term; can be used in political, social, or personal contexts.
彼はその政治家の熱心な支持者だ。
He is an enthusiastic supporter of that politician.
この計画には多くの支持者がいる。
This plan has many backers.
ファン is a casual loanword for an admirer or enthusiast, often of entertainment or sports figures, whereas 支持者 is more formal and implies active support.
Compound of 支持 (support) + 者 (person). The word is a straightforward Sino-Japanese formation.