protagonist

Compared meaning: a supporter of a cause (noun).

比较义项：某项事业的支持者、拥护者（名词）。

• friend

  The mildest and most general term; simply someone who supports a cause or person, without implying public advocacy or strong action. Common in the collocation 'a friend of' plus a cause or place.

  语气最温和、最笼统的词，仅表示支持某项事业或某人，不含公开倡导或积极行动的意味。常见搭配为“a friend of”加某事业或某地。

  The senator has long been a friend of environmental causes in Congress.

  这位参议员长期以来一直是国会中环保事业的支持者。

• protagonist 本词

  A formal, somewhat literary word for a leading supporter who actively drives a cause forward; less common in everyday speech, often used of historical or intellectual movements.

  正式且略带文学色彩的词，指积极推动某项事业发展的主要支持者；日常口语中较少使用，常用于描述历史或思想运动中的人物。

  He became a leading protagonist of the reform movement in the 1960s.

  他在20世纪60年代成为改革运动的主要倡导者。

• champion

  The strongest word: someone who fights publicly and persistently for a cause, often visible or vocal; can also be used as a verb, 'to champion something'.

  语气最强的词，指为某项事业公开、持续奋斗的人，态度鲜明、常引人注目；也可作动词使用（to champion something，意为“为……而奋斗”）。

  She has always been a vocal champion of workers' rights.

  她一直是工人权利的坚定拥护者。

• booster

  An informal word, especially in American English, for someone who enthusiastically promotes and encourages something, often at a local or grassroots level; lighter in tone than champion.

  较为口语化的词，尤见于美式英语，指热情地宣传、鼓励某事物的人，常带有地方性或基层色彩；语气比 champion 更轻松、不那么严肃。

  He's a big booster of the town's new sports arena project.

  他是这座小镇新体育馆项目的热心支持者。

Friend, protagonist, champion and booster mean a supporter of a cause. Friend is mild and general. Protagonist is formal, a leading supporter driving a cause forward. Champion is the strongest, someone who publicly fights for a cause. Booster is informal, an enthusiastic promoter.

这四个词在表示“某项事业或主张的支持者”时意思相近，但语气和正式程度不同：friend 较为温和、笼统，protagonist 正式且偏文学化，champion 语气最强、常指公开有力的倡导者，booster 则较口语化，多指积极热情的宣传者。这里不比较 friend 的“朋友”、protagonist 的“主角、主人公”、champion 的“冠军”，以及 booster 的“助推器、加强针”等其他常见词义。