Usually the main villain of the season/show. Alongside the "Bigger Bads", they are the most difficult to defeat or even kill.
They are the main villains of the anime series or season that plan to defeat the hero(es) and can serve as a main part of the entire plot. They can also summon and/or control other enemies and order them to defeat the hero to either get rid of them or serve as a test for the hero to face the Big Bad.
They are villains that happen to be even stronger than the Big Bad, but only harbor a small part of the story at most times. In order for the villain or villainess to become a Bigger Bad, they must be alongside a Big Bad for the majority of the story and serve as a minion of the Big Bad until said the latter is either defeated or killed in combat. They are also determined to kill off the hero once and for all and will most likely take over the main goal that the Big Bad wanted.