Grant is a long time NCRranger, best described as the "shoot first, never ask questions" type of soldier, someone who doesn't mind going into combat, so long as his soldiers shoot alongside him. He was injured at one point, which may explain his aggressive nature. He also seems to enjoy (and even volunteer for) more dangerous missions.
xbox360ps3 If you inform Ranger Grant in person after killing the Legion Sniper on the tower during the quest You'll Know When it Happens, or if you choose the "goodbye" conversation option before the quest, he will sound as if he is talking to you through a radio even though you are standing in front of him. [verified]
pc Sometimes he says that they're preparing to protect the president, and the only dialogue option is "Goodbye". To fix this, reset Ranger Grant by typing prid 0013721F then disable and last enable. [verified]
pc When you say "I have to go.", he will say bye in a different voice. [verified]