A person who feels envy toward someone else; someone who envies another person.
From 'envy' plus suffix '-er' (one who does). This follows the standard English pattern of adding '-er' to verbs to create agent nouns naming people who perform the action.
While 'envier' is grammatically correct, it's rarely used because we usually just say 'someone envies' rather than calling them 'an envier'—showing English prefers describing the action over naming the person.
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