Under

Under (アンダー), also known unofficially as the Negator Hunters, are a group of Negators that oppose The Union. Due to their pasts, they seek revenge against the world and hope to build up an army of strong Negators to take it over.

A small group of them, led by Rip, serve as the antagonists of the Unrepair Arc.

It's later revealed that Billy is the leader of the group and plans to work with them to complete the quests and kill God, as he takes the Roundtable and Apocalypse to them.

Trivia

 * The logo for the group resembles the Kanji for "Un" but upside-down.