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."