Deis is a recurring character in the Breath of Fire series. She is an immortal naga, and while she is one of the heroes, she is said to have some connection to all that is evil in the Breath of Fire series. Aside from Ryu and Nina, Deis is the only other character to have appeared in multiple Breath of Fire games and have any big significance to the plot.
Because of her immortality and several other hints throughout the games, Deis is assumed to be the same person in every game, whereas Ryu and Nina are new incarnations every time. Breaking with tradition, Deis makes no appearance whatsoever in Breath of Fire: Dragon Quarter, one of many reasons why it was unpopular with fans. However, early concept art of a different character, Lin, depicted her with a striking resemblance to Deis. In her in-game appearance, Lin actually resembles the character Katt from Breath of Fire II, and is assumed to be a descendant of Katt, with no relation at all to Deis.
In Japan, the character's name was always "Deis", though in the English versions of Breath of Fire and Breath of Fire II, she was renamed "Bleu", meaning many players did not realize the significance of the Deis character in Breath of Fire III, who didn't look anything like her previous incarnations.