Did you guys read the ghost fragment Cayde-6 grimoire? He had a Fallen ally even if for only an hour or so. They made a final stand near a crashed jump ship and when she became wounded from fighting a Hive Knight, she slumped against the ship hull and threw him her last shock pistol and provoked him to kill her when he went pick it up. From my understanding, she was wounded with no way of recovering and wanted him to euthanize her but couldn't bring herself to ask and didn't want to put him in an awkward spot. I thought grimoire was really deep and felt Cayde-6 should have rescued her and brought her (against her will) back to the tower for research. From there she would slowly and regrettably become an ally and since she didn't belong to an actual Fallen House (her pack was exiled and died at the hands of the hive) she wouldn't be a traitor. She could be a rare vendor and could stand near the comms to left or right of the vanguard table. She could offer Rare ghost shells, and a weekly random assortment of legendaries that require reputation. Such a missed opportunity Bungie :(
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The Fallen and Cabal are not with the Darkness. They are similar to us... Trying to escape it. The Fallen are roaming space pirates, while the Cabal are running from it, and Earth just so happens to be in there path. The Hive and Vex are another story. The Vex worship the Heart of the Black Garden, which is a sort of conduit for the Darkness. The Hive are literal soldiers of the Darkness...embracing it.