400

The Volunteers handler in the beginning of the game. He gives out a few missions that are needed to advance the story and unlock certain aspects of the game. He works for New Light.

Spoilers:

400 would later send the volunteer on a mission not sanctioned by New Light. To recover certain documents from a installation and send them to him via mailsystem. The volunteer would later experience a voice speaking to him and would contact 400 afterwords informing him of this. 400 claims the voice also spoke to him, and also describes what the documents that were recovered held.

He spoke of his wife and daughter, who official were listed as dying in some eruption that occured. Though the documents reveal that his wife and daughter was sent to a secret lab and experimented on. They both die from this, though her daughter, named Lily, appears as a ghost that haunts the island and torments anyone she finds. This is most likely the little girl ghost that spawns for a few seconds to scare the player at certain times. After this he states he must "find answers" and leaves the volunteer, abandoning him on the island.

400 would be replaced by 11, who contacts the volunteer not long after and informs him that 400 has disappeared and questions the volunteer over the mission 400 sent him on. The volunteer feigns ignorance as he has a suspicious feeling theres more to this then New Light and 11 is letting on.