Emmet needs Rosalie. He loves her. You'd be pretty grumpy if 你 were gang raped, inches from death, then transformed into a monster. But Emmet is not the kind of guy to be pushed around - he loves her for who she is, bitterness and all.
你 are so right, they are together for show, his layed back joking side is perfect for bella, rosalie and edward perfect for each other, they think the world revolves around them, and rosalie is just with emmett cause ed 说 no