IPA: /ˈæd.və.keɪ.tər/
KK: /ædˈveɪkətər/
A person who supports or argues for a cause, policy, or idea.
As an advocator for environmental protection, she tirelessly campaigns for sustainable practices.
Advocator is formed from "ad-" (meaning to or toward) and "vocare" (meaning to call). The word refers to someone who calls or speaks on behalf of another, essentially advocating for them.
Think of someone who 'calls' ('vocare') for you or speaks 'to' ('ad-') support your cause — that's what an advocator does.