When it comes to describing a person who appears determined and steadfast, the phrase "with set jaw" can be used to great effect. However, using this phrase repeatedly in your writing can become tedious and detract from the impact you're trying to create. Instead, try using synonyms such as "resolute expression", "determined countenance", or "unyielding demeanor". Another option could be to describe the physical actions that exemplify the phrase, such as "clenched teeth and a firm jawline" or "a tight-lipped expression". By using varied and descriptive language, you can paint a vivid picture of your characters and their emotions without relying on repetitive phrases.