In Taylor Swift's “august,” the narrator's summer romance is with James.
James is the 17-year-old boy at the center of Folklore's fictional teenage love triangle. He is already connected to Betty, but he becomes involved with the unnamed narrator during the summer. When the season ends, James returns to Betty, leaving the narrator to grieve a relationship she hoped might become real.
The three connected songs present different viewpoints. “cardigan” is associated with Betty, “betty” presents James's perspective, and “august” follows the unnamed girl. Swift described the story as fictional rather than a direct retelling of her own romantic history.
Listeners sometimes call the narrator “Augustine” or “Augusta,” but those are names Swift said she used in her imagination; the song never explicitly names her. James, by contrast, is clearly identified in the broader Folklore narrative, making him the canonical answer.