。
A self-made success story.
Literal
Success-story-within [possessive-の] person.
立志伝中の人 means literally 'a person within the success story' — someone whose life reads like a rags-to-riches tale. 立志伝 (success story/biography of a self-made person) + 中の (within) + 人 (person). A compact literary expression.