Rockstar is a confident member of Shanks' crew who seems to have a lot of pride. When he was insulted by Whitebeard's men, he wanted to fight them because they had damaged his honor. He also tried to used the strength of his captain to back up the importance of a letter he delivered to Whitebeard. Shanks sent him to deliver a message to Whitebeard, which the pirate captain promptly ripped up, going along with Shanks' prediction anyway. Even so, Rockstar wanted to fight Whitebeard to reclaim his honor, but Shanks told him it was no big deal and to pull back.

Source: One Piece Wikia