IPA: /kruːˈseɪdə/
KK: /kruːˈseɪdə/
A person who is involved in a campaign or movement to promote a cause, often with a strong sense of purpose or dedication.
The environmental crusader worked tirelessly to raise awareness about climate change.
Crusader → It is formed from "crusade" (from Old French *croisade*, meaning a campaign marked by the cross) and "-er" (meaning a person who). A crusader is a person who participates in a crusade, particularly in the historical context of the religious wars in the Middle Ages.
Think of a person who is on a 'campaign marked by the cross' — that's what a crusader is.