leading advocate; main proponent
Refers to the person who takes the lead in advocating a cause, proposal, or movement.
He is known as a leading advocate of the environmental protection movement.
The main proponent of the reform plan was a young lawmaker.
Compound of 主 (shu, 'main') + 唱 (shō, 'advocate') + 者 (sha, 'person'). The exact historical derivation is uncertain, but the word follows a common Sino-Japanese pattern for 'person who does X'.