From Portals of Phereon Wiki
For the original unique town NPCs (Lumira, Sylvie, Castalia, Aila, Flora, Mya), go to the Town NPCs page.

Unique characters are distinguished by their Unique special trait and special art. They may be obtained through various events and conditions, providing consistent traits, skills, abilities and special game mechanics. As a rule, all unique units gain a bonus to their stat growth. In combat, they can't be recruited or mind-controlled, and they can't die (you can lose them, but they don't trigger death effects). Certain uniques have additional special events that make them stronger.

You can find various events related to them on this page

Some Unique characters can be fused without losing them, for more information check here