Banjo has a secret obsession with Bottles the mole.

You will notice when Bottles speaks with Banjo during the opening cutscene of the game that he has not met Banjo before that point. However, Banjo certainly knows who Bottles is as he has a massive portrait of him hung up in his house, the biggest portrait in his house in fact. I theorise that Banjo has had a secret obsession with Bottles for a very long time and due to some neuroses had not been able to introduce himself to him prior to the beginning of the game.