Murmur is a Cabalist NPC that will be found at each station you visit throughout the game. He appears first during the tutorial mission and is involved in the main storyline until the very end. He is a guide that knows a great deal about the events in the game and can help you out with each quest by giving you completion suggestions.
In game description:
It would appear Murmur is willing to share what he knows of the world - a vast collection of inane facts and strangely pointed observations - if only to participate in simple conversation.
His earnest effort at companionship in a time when few men offer hands of friendship not motivated by hidden agendas is a welcome rarity.
But where he gets his info, no one knows.