The Wind Runner starts with 10k HP and it costs 3500 exp to get to 1st star which is then 20k HP.

I would suspect the experience doubles each time similar to dragon with HP going up by 10k increment.

so 7k exp = 2nd star
14k exp = 3rd star
28k exp = 4th star
56k exp = 5th star
112k exp = 6th star
224k exp = 7th star
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