Stone is Rock's brother and combat partner, taking the form of his weapon in battle.
He's fairly numb to the life of a soldier, considering his role is more to do with intelligence collecting. He's very inquisitive like his brother, though he's less prone to flaunting his intelligence over people.
He trusts in Rock completely, though he doesn't know everything that he has going on.

Stone's development is fairly absent for this season, though I'm laying the foundation for what comes later.