Vaelia was once a gladiator at the Black Dragon Tavern. Ariel bought her after Vaelia won a hard battle and moved on to try to kill the arena guards after disposing of her first enemy. Ariel was impressed, and offered to save her from the harsh life of a gladiator, in exchange for the human's loyalty. Vaelia agreed.
Vaelia was known as Vaendra in the earliest chapters, that name is now being written out of the series entirely.