Does he have PTVD caused by some horrifying experience in battle or even something involving family deaths? Or do you think he just prefers not to talk? If so, then why does he prefer not to talk? Anti-Social? Let's hear some possibilities. Be creative.
[Edited on 08.29.2011 10:29 AM PDT]
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Without considering the fact that Bungie said they purposely did that so the player can "fill" Rookie's shoes, then I'd say he simply likes to be silent and out of everyone's way.