What are his key words? Or what is his nature?
These are related questions, but not the same ones.
Some of his key words are more important for their role in the play than for what they say about his nature. For example, his first lines introduce the sergeant who was at the battle where Macbeth distinguished himself. These lines matter, and are dramatic, but just about anyone could say them.
Likewise, his words when he learns of his father's death are important; they tell us that he and his brother are there, and that they are mostly aligned regarding reacting to his death. However, those too don't say that much about his nature.
Where you see his nature are a few places: in his actions (he leaves Scotland pretty quickly), in his exchange with Macduff in England (where we learn he's pretty manipulative, ready to test Macduff's loyalty), in his sympathy to Macduff in his loss, and in his opposition to Macbeth.
He is complex, brave, and, in the end, noble. He'll be a good king.