Portraits live or die by light, so structure the prompt like a lighting brief: who the subject is (age, one or two distinguishing details, wardrobe), the light source and its direction, the background, and the lens look. “Portrait of a woman with curly gray-streaked hair, backlit by low golden hour sun, 85mm f/1.4 bokeh” gives the model everything it needs; “beautiful realistic portrait” gives it nothing. Named lighting setups work too — Rembrandt light, beauty dish, window light, color gels — because the models were trained on photography that uses exactly this vocabulary.
Keep subjects generic (“a young man”, “an elderly potter”) rather than naming real people — the models refuse celebrity likenesses. If you need the same face across a whole series, generate one portrait you like and reuse it as a character reference on Nano Banana 2 or Pro; that workflow is covered in the character consistency guide linked below.