Someone who's on your side, who supports you wholeheartedly, is your sympathizer.

A sympathizer wishes you the very best, especially when you're going through a hard time or working to achieve a goal. Your best friend might be your number one sympathizer, agreeing with your opinions and backing you up when you need support. Key to the noun sympathizer is the verb sympathize, "agree with a sentiment" or "have fellow feeling." Both words come from the Greek sympathes, "affected by like feelings," a combination of syn, "together," and pathos, "feeling."

Definitions of sympathizer
  1. noun
    one who commiserates with someone who has had misfortune
    synonyms: comforter, sympathiser
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    Job's comforter
    someone whose comfort is actually discouraging
    type of:
    a person who communicates with others
  2. noun
    someone who shares your feelings or opinions and hopes that you will be successful
    synonyms: sympathiser, well-wisher
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    bleeding heart
    someone who is excessively sympathetic toward those who claim to be exploited or underprivileged
    fellow traveler, fellow traveller
    a communist sympathizer (but not a member of the Communist Party)
    type of:
    admirer, booster, champion, friend, protagonist, supporter
    a person who backs a politician or a team etc.
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