Justin Bieber (also known as Justin Beaver or more colloquially JB) is a minor character in the SMG4 series.
He is a Canadian singer, best known for his semi-infamous song Baby. He has gained many fans and haters alike over social media, and to this day is a controversial figure.
He appears in some of SMG4's bloopers. He originally auditioned for the role of Mario, as seen in SM64: Meet the Mario.. Meanwhile, his songs and pictures of him have been used as weaponry due to how atrocious everyone finds them. Nevertheless, many characters, including Toad, SMG3, Bonnie the Bunny, and especially Bowser are Bieber fans.
He is also mentioned in the WOTFI Rap battle 2016 where SMG4 says "go back home and hump some Justin Bieber" to Bowser. After this mention, he was dropped off completely from the series for unknown reasons, but the most likely answer is that the internet have already move on from putting him as the punching bag.