Based on the information provided in the reference, the character identified as Ron Weasley's girlfriend in Harry Potter and the Half-Blood Prince was Lavender Brown.
According to the reference:
Her first major role was as Ron Weasley's girlfriend, Lavender Brown, in Harry Potter and the Half-Blood Prince.
This reference highlights Lavender Brown's role as Ron Weasley's girlfriend within the context of the Harry Potter and the Half-Blood Prince story. While this particular relationship is known to be Ron's first significant romantic involvement portrayed in the series canon, the provided reference specifically states that the actress's "first major role" was as Ron Weasley's girlfriend, Lavender Brown, in that specific film/book adaptation, rather than explicitly stating that Lavender herself was definitively Ron's first ever girlfriend within the broader narrative history.
Therefore, based directly on the provided reference, Lavender Brown is named as Ron Weasley's girlfriend in Harry Potter and the Half-Blood Prince.
