cassian and vesper both have indeterminate pasts and no one quite knows who or what they are. cassian believes he is human, having been brought up by human parents, but is actually a Bright-Eye, an alien race whose most distinctive characteristic is mild bioluminescence, especially in the eyes (hence the name). He was abandoned when his parents learnt he was Blessed by the gods and adopted by his current family, who didn’t tell him the truth until his 18th birthday.
vesper is a little more complicated. he was also dumped by his family when they learnt he was Blessed, but at an orphanage. when everyone finds out cassian is a Bright-Eye it becomes clear he has Bright-Eye blood in him, but since he is so much more luminescent than cassian, that can’t be all. add to that his lavender coloured skin, silver-threaded hair and pointed ears, and he’s an utter mystery for a long time. eventually it comes to light that he is half Bright-Eye, but his mother was a star spirit inhabiting the body of a forest elf, thus explaining the bioluminescence, as well as the powers he has that come from his patron, the goddess of stars.