So according to this gamefaq:

http://www.gamefaqs.com/xbox/538752-enclave/faqs/20975

There is an extra character when you collect all the money on the light or dark campaign.

After looking for all the 13000 gold of the dark campaign, the extra character (Feticia) is not unlocked, i cant find it anywhere.

So... anobody has it? Maybe its just for the xbox version :(

She (and the 9320 Battle Droid for the Light Campaign) are both available in the GOG version, I unlocked both. They are available only in the profile in which they were unlocked though - you have to load up that profile and re-play a mission to have that character option (it's not possible to unlock them without completing every campaign mission and every level of every optional arena, because they are so overpowered that it would be ridiculous to allow you to use them for anything other than a high-speed, high-explosive rerun).