He chose Major Graves for his second (that name is not right, but it's close enough; I don't remember the Major's name).

Graves came over to instruct Joe in the duelling art.

He had been a Major under Walker, the "gray-eyed man of destiny," and had fought all through that remarkable man's filibustering campaign in Central America.

That fact gauges the Major.
To say that a man was a Major under Walker, and came out of that struggle ennobled by Walker's praise, is to say that the Major was not merely a brave man but that he was brave to the very utmost limit of that word.

All of Walker's men were like that.