I wonder what the fallen guardians were like.

No, the Fallen did not have Guardians. That’s a common misconception, but the lore tells us about it. The Traveler visited many species, some of which are listed in the Books of Sorrow. The Traveler helped all these species create Golden Ages filled with technological advancement, but it never gave anyone the Light. There were some situations where it used the Light, like to terraform worlds or create the Gift Mast, but no species was ever given the gift of the Light. Then the enemies of the Traveler would always arrive and the Traveler would flee, leaving the species it visited to die. This changed when the Traveler and Darkness came to Humanity. This time the Traveler didn’t flee and it lashed out at the Darkness, repelling the Darkness and falling into a slumber. This was when the Traveler created the Ghosts and gave Humanity the gift of the Light to its Chosen, the Guardians. No other species was ever given the Light.

I don't remember the Traveler fleeing at any time except for the first time at fundament, every other time the Traveler had already completed it's work and left, maybe I missed some lore.

You’re right! But the Traveler also fled from the Eliksni home world, shown from a handful of lore excerpts on the Whirlwind and Chelchis. And the Dreams of Alpha Lupi spoke of how the Traveler spent all its existence fleeing from the Darkness chasing it, if I remember correctly (I haven’t read the Dreams of Alpha Lupi in awhile, lol). Either way, the Traveler never stayed around to confront the Darkness, so I guess that doesn’t change anything. Thanks!

<Except maybe the Ammonites. At least they were able to use Light-based weapons. Still, even if they had Light-based abilities, there never was a Ghost until after our Collapse.>