志愿者
zhì yuàn zhě · HSK 3
volunteer
🙋 志愿 = will + wish; 者 = -er, so 'wish-doer' = volunteer
志 (will/aspiration) + 愿 (wish/desire) combine into 志愿 'aspiration, willingness'. The suffix 者 marks a person who does something, like English -er. So a 志愿者 is literally a 'willing-wisher', i.e. someone who volunteers.
愿 — 原 (source) + 心 (heart)
A semantic-phonetic compound. The heart radical 心 carries the meaning of inner feeling or wish, while 原 (yuán, 'source/plain') supplies the sound. Together they depict a wish welling up from the heart, giving the senses 'to wish, to be willing'.
者 — old/elder 耂 + say 曰
Originally depicted sugarcane or a similar plant, but the form was reanalyzed as 耂 (old) over 曰 (say). It became a grammatical suffix meaning 'one who…' or 'the…thing,' nominalizing verbs and adjectives. Today 者 mainly marks agents or categories, as in 学者 'scholar' or 记者 'reporter.'