There was one First Sergeant for each company of soldiers. His function was key to maintenance of discipline and morale in the company.
He is wearing a 9-button frock coat with first sergeant chevrons. The trim on the coat and chevrons are light blue in color. His headgear is a forage cap (kepi), but we are unable to see if there were any insignia on the top. His pants are light blue and appropriate for a private rather than a noncommissioned officer. These lack the regulation one and one-half inch blue stripe that indicate a sergeant. Trousers were issued without stripes and the stripes there added from company stocks in the field. It is possible that many sergeants did not wear trousers with stripes in the field.