James is Betty's former boyfriend in the storyline of Taylor Swift's "august."
The song presents a fictional young woman, sometimes internally called Augustine or Augusta by Swift, who has a summer romance with James. James is already connected to Betty, and the relationship eventually returns to Betty after the summer ends.
This narrative is spread across three Folklore songs. "Cardigan" is told from Betty's perspective, "Betty" follows James's perspective, and "august" gives voice to the unnamed girl involved in the summer affair.
The narrator of "august" is not officially named in the song itself. That is why Augustine is not the answer to this question: it refers to the narrator, while James is the boy at the center of the triangle.