Ennis is Firo's lover.
She's the first homunculus created by Szilard, an heartless business-man that want to reach all power and informations about immortality, and for this reason he uses Ennis like a weapon, leaving to her the assassinations and the dirty jobs. But fortunely, Ennis meets Firo, the guy of her destiny, the person that saves her from a tragic death. Firo falls in love with her since the first moment, and Ennis too, but the difficult situation fetters the expound of their feelings.
In the anime they become a couple in the end of the series (last 3 episodes from DVD excepted), but in the novel they get married, even if after much years.
Ennis thoughts to doesn't have feelings but Firo proves to her that every person has an heart, an heart that can beat for another one.