On rare days, usually when she has to venture out into the field, Nell wears her hair in a ponytail. Callen isn't terribly sure he likes her choice of hairstyle on those days because he can't really tangle his fingers in it when they sneak kisses over lunch of during breaks.
Although.
He does so enjoy the sexy tousled way it falls around her shoulders when she deconstructs the tight style. Maybe, just maybe, he could persuaded to enjoy the occasional ponytail.