WASHINGTON — The leader of the U.S. delegation to this week’s talks on normalizing relations with Cuba has a reputation for determination and expertise at the State Department, where she flouted custom with an unlikely rise from rank-and-file civil servant to a role that’s been described as “horse trader of the Americas.”

Diplomats and Latin America specialists describe Roberta Jacobson as a hard-charging stateswoman who tempers a steely professionalism with humor and warmth. As the assistant secretary of state for Western Hemisphere affairs, she oversees 10,000 personnel in 30 countries, is the first woman to hold the job and was the State Department’s first regional deputy assistant secretary to have risen to that rank without first being a foreign service officer.

Across the board, diplomats described her ascent to assistant secretary from the civil service, not the more specialized foreign service, as “exceedingly rare” and “practically unheard-of.”