Drien seems to be one of the most important characters in the history of Dragaera about whom we know the least.
It may be that Drien was the other sibling of Kieron (that is, not Dolivar) who has reincarnated as Aliera e'Kieron. At least one hint in this direction is Aliera's regression-memory of being a Shaman when she was Kieron's sibling. Alternatively, it is speculated that Drien's soul inhabits Pathfinder (and a hint that might support this is the ambiguous sense of gender that Vlad notes in Pathfinder's aura).
Drien seems to be a figure with a penchant for duality. Drien was both male, and female, both Warrior and Shaman, etc.
Is it possible that Drien in fact represented two separate souls in the same body? This could mean that Drien's soul inhabits BOTH Aliera e'Kieron, and also Pathfinder... And would serve to explain the connection between the two.