In Act 1, Scene 2, what does the sergeant specifically report about Macbeth? What does this news reveal about Macbeth's character?
The sergeant specifically reports that Macbeth is a brave warrior.
For brave Macbeth--well he deserves that name--
The text also specifically indicates that Macbeth gave the killing blow to Macdonwald. It wasn't just a simple stab of the sword either.
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements.
That means Macbeth sliced open Macdonwald from his belly to his face. Then Macbeth cut his head off and put it on a stake for all of the army to see. That's brutal.
The battle was not won at that point, because a fresh assault by the enemy began. It seemed as if Macbeth's army would falter, but the sergeant says that Macbeth and Banquo redoubled their efforts from some unknown pool of energy reserves.
If I say sooth, I must report they were
As cannons overcharged with double cracks, so they
Doubly redoubled strokes upon the foe:
The information given about Macbeth in this act helps establish that Macbeth is a brave, courageous, and strong man. It also tells the reader/audience that Macbeth is familiar with killing and bloodshed. Later in the play, when Macbeth starts killing anybody with a claim to the throne, the audience is able to relate it to the killing propensity that Macbeth showed in battle.
He reports that Macbeth is a worthy soldier, both courageous and honourable. He can carve a man from "nave to chaps" (belly to head, essentially) with no guilt or hesitation, much less a handshake or a greeting first. He kills Macdonwald and puts his head on a stake. Rather gruesome. When the stakes are not in his favour and the Norwegians attack, he doesn't give up hope, either. He doubles his efforts and kills many more.
This tells us that he is ruthless, courageous in battle, and is willing to do whatever is necessary to gain victory. These characteristics will surface again when he "battles" to attain the crown. He will kill anyone who stands in his way...quickly, and without mercy.