It's safe to say that while Ratchet is the hero of the game, he is not a nice guy for the first two-thirds of it. His reasons for going off on the adventure are purely self-serving (i.e. he wants to get off of Veldin to have some fun, with the Blarg just being an obstacle) and he acts rude and abrasive towards Clank, who is dead set on trying to stop Drek's plans and save the galaxy. Once Captain Qwark betrays him, he becomes a borderline Hero In Name Only due to his obsession with getting even with Qwark taking priority over saving the galaxy and souring his relationship with Clank. He eventually comes around once he sees the consequences of his actions unfold (i.e. Oltanis getting ravaged by the Blarg because he was too busy fighting Qwark when he could have stopped them) and learns to be nicer to Clank and more responsible from there on.