How to unlock the Fallen playable guardian and 4th character slot: Find live ghost drained of light living off of ether, take to Variks in the reef. He inspects it overnight and gets an idea to infuse it with light to restore its abilities. He give you a sword that absorbs your light whenever your super gets a full charge. (Drains your super). 3 supers charge the sword that you return to Variks to infuse into the damaged ghost. He attempts the infusion but 'fails' the light restores the ghost but doesn't replace the ether making to ghost useless for human guardians. After reset Variks summons you back as he has an idea for the ghost. You must find an honorable fallen warrior and defeat him so the ghost can turn him into a guardian. He has heard rumors of a fallen vandel on earth that even guardians respect and speak of. After defeating the Vandel, and escorting him to Variks, Variks says the new Guardian Kell will need a ship and a home to operate from. He sends you to clear out and claim the only ship he knows of sitting on Venus. And to clear the empty tower the postal officer always talks about. At this point Variks has enough info to fix and infuse most damaged ghosts due to the mixture of ether and light. Now as the final step you just have to steal all the damaged ghosts the speaker is keeping locked in the tower.