1)The traveller contains the light, which is essentially the minds of every human left in the Universe, or maybe has millions of humans in stasis. 2)Guardians that have lost their light become corrupted, and their minds transferred into the different alien races, hence a similarity in weapons and playstyles. 3)The main character is amnesiac due to either the Traveller purposefully withholding key information, or the mind transfer wasn't 100% successful due to the Travellers weakness. 4) The awoken race, are minds(souls) that have truly awoken, remembering their past life, the major war, just about everything, making them an unknown factor, as they can freely choose to side with themselves, the darkness or the light. 5)If the traveller is purposefully withholding information, then you can think of the Ghosts as an accessory,(from Freedom Wars) or an inspector. Basically their job is to aid you, whilst making sure you don't fully wake up, the mission where a computer says Hello Dr Shim, but your ghost basically wants you to ignore it for example. Lastly, the Traveller might be from the future, and due to a battle, it couldn't get far enough back in the past to truly help mankind, and the Darkness could be from an alternative universe, the Ying to its Yang, corrupted, thinking the only way to save the universe is by wiping out humanity.