Alrighty, after finishing 2 NG characters and working on NG+ now, here are my thoughts.
Vendrick:
Acquired the original 4 great souls and used them to build Drangleic. After this, Nashandra showed up to try and abuse his power. She convinced him to attack the giants for a prize. After the giant war, the curse started, and Vendrick and his brother Aldia began doing everything in their power to defeat the curse. After so much time, Vendrick discovered that Nashandra was behind the curse and so he left. During this quest, one of his chosen champions, Velstadt, turned traitor, revealing himself to be corrupted by the dark. He imprisoned Vendrick and separated him from his soul. Until the PC comes through, Velstadt guards Vendricks tomb, ensuring that no one can acquire the soul of the king.
Nashandra:
A piece of Manus, she is the reason the curse begins again. Her desire for the age of dark corrupts most of the population of Drangleic. She corrupts Velstadt and schemes to imprison Vendrick because he doesn't want to continue the cycle by either lighting the flame or ushering in the age of dark.
Shanalotte:
The last firekeeper, but someone also born of dragons. Shanalotte guides the PC to the dragon Aerie, but for what purpose? She works only for herself in an attempt to defy her own fate. Speaks in half-truths, but never straight out lies.
The Duke:
It's pretty clear that either the Duke or Freja is a reincarnation of Seathe. Seathe was known to be searching for immortality. In the first game, its implied he failed, since he was searching primarily for the scales of immortality, the natural way the dragons did it. But it seems he succeeded, just not the way he intended. I'm inclined to think that Freja is actually Seathe and just corrupted Duke Tseldora to let him run amok with his spiders in that area.