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.

The Negator War Arc would retroactively reveal that Billy, Tella, and Creed are the founding members of the Under. Due to events playing differently in the 101st loop, Under no longer exists, as all three founders have peacefully joined the Union following the aversions of their tragedies.

Overview
Under was formed by three survivors in a war over Disc, Billy, his communications officer Tella, and the enemy leader Creed, who were left to die thanks to their Negator abilities awakening and the superweapon being stolen. After Billy joined the Union for his own plans of a better world, seeing the people Juiz had recruited made him doubt they could slay God, and so the three of them worked to gather forces as Billy kept up appearances so to take all their Negator abilities with his Unfair.

Trivia

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