Ok so I already posted my theory on another user's blog but I thought of doing one my own....
Here's my thoughts on Setrakus Ra;
From what we say in the Rise of Nine, towards the end, Ra revelead quite a bit of his powers, and for those who don't recall;
-Telekinesis -> Used it to lift Six to the ceiling
-Power Drain (Don't know it's proper name) -> He shoots out a bolt of electricity across the room, and thus somehow shuts down the gardes abilities
-Shapeshift into Anybody ALIVE! -> He shapeshifted in power of six and rise of nine, both times as Sarah and once as Six towards the end of RON.
-Communicate with the Garde in their Dreams
-Enhancement - He's strong as he was able to fight both pittacus and Six and fast enough to beat Nine.
-More Unknown abilities
So what I think of Ra is that he's probably a Loric, who was banished from Lorien because he probably thought of world domination or something along the lines. And here's another theory- remember in power of six when they crawl down to sam's father secret hideout and they find a HUGE skeleton of someone wearing the loric pendant? What if that was the REAL MOGODORIAN Setrakus Ra and the bad loric killed him and assumed his place?
Ofcourse, there are contradictory statements from you guys I know, like why would a loric shapeshift into a mogodorian and didn't I just say that he can shapeshift into someone alive? but these are just theories :)