Ken walks into Kai's room waiting for Rex to wake up, with his physical strength and power he has the potential to be a dragon but he's not sure, by now he should be showing signs of what kind of immortal he is.
If he is not showing that makes Ken's options very simple, Witch, Luxor, syphoner, and seer. He's hoping with everything he's got Rex is a Luxor, they are an extinct race of immortals who can kill any type of immortal.
Flaw in this however is that they can get killed by any immortal either, however, if a Luxor is tied to a Luxar they are basically unkillable. That's how the Luxor's who are still around now have survived.
They don't bother anyone who doesn't bother them. They don't get involved in senseless wars either, they've got nothing to lose, if the entire world was extinct they would be one of the top four and only four races of immortals to survive.