Manasseh is a notorious King of Judah who definitely gets filed in the narrator's "bad king" category. He engages in all manner of prohibited religious practices and basically winds up being the last straw, as far as God is concerned. The disobedience of Manasseh provokes God into finally ordering the people of Judah into exile, allowing the Babylonians to invade and destroy.
Even though the righteous king Hezekiah is his father, Manasseh becomes wicked—following a typical pattern where latter kings, whether Israelite or non-Israelite, tend to be worse than their predecessors (the Babylonian Belshezzar and the Greek Antioch Epiphanes both being good examples of decline). He reigns a record-setting 55 years in Judah, does abominable things like sacrificing one of his children in fire, and practices divination and sorcery, before he eventually dies.
Appropriately enough—given that he makes Judah forget God—his name means "causing to forget."