The White

The white are a group of souls tired of the suffering of humanity who seek to put an end to it, and will seek strong-willed people close to "Messiahs" that they can use in a way to fulfill their goals. However, if they try to refuse long enough, they will try to force them to cooperate... by force.

Season 6
The White are the enemy that Heavy Machinery's erratic actions through the season were fighting, denying their chances to get close to the Messiah and ultimately forcing a stalemate that drove them away, saving the universe.

They also might've originally appeared to Kadoya Tsukasa in his original timeline and fought him after he had tried to use Delta's Alien Fax Machine and failed. However, what happened after that is unknown, they may have won and "purified" that universe, or they might have won and denied them.

Trivia

 * The White were supposed to appear in Season 6, but were discarded due to being too "high stakes".