Betty is James's former love in Taylor Swift's “august” storyline.
The song is part of Folklore's fictional teenage love triangle. “August” follows an unnamed girl who has a summer romance with James, while James remains emotionally connected to Betty. “Cardigan” presents Betty's perspective, and “Betty” presents James's.
The unnamed girl is sometimes called “Augustine” by fans and in commentary, but that name is not the song's official character name. The song itself does not identify her with a conventional personal name.
Swift described the three songs as connected fictional perspectives rather than a direct account of her own life. That distinction matters because listeners sometimes try to map the characters onto real people, even though the narrative was deliberately imagined.