Larian focuses its story efforts more on dialogue and quests instead of an overall plot or character backstories.
Don't expect the characters to have a backstory. The only one of the previous Divinity protagonists who had a backstory was the Death Knight in Beyond Divinity. That's pretty typical of so-called "Western" RPG's, they often have protagonists without a history, to let the player make up their own characters through their actions.
The backstory has changed several times since Original Sin was announced. First, there was talk that Roderick was a guy who had spent years undergoing torture (I think because he had some inherent regenerative power, and they were experimenting on him), and Scarlett had died but was recently resurrected somehow. (I think that somewhere the idea wes mentioned that she was reborn and seeing everything through the "Eyes of a Child", hence the working title of the game.)
Then during the Kickstarter, there was a story thing where Roderick and Scarlett were the sole survivors of a massacre at the Source Hunter Academy, and Zandalor recruited them to track down and learn how to use the Source.
Now they've been sent on an official mission, with orders and everything, and I have no idea what the plot hook will be after Jake's murder.
I still have no idea what Source Magic is. In Divine Divinity, it was used for the powers of good for healing, then earlier in the backstory it was the powers of nature, four elements stuff, now I think it's something like evil blood magic or whatever. It keeps changing.
