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. They serve as a major antagonist for most of the series up to the Spring Arc, following which, they join with the Union to try and prevent Ragnarok.

Trivia

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