Saishuu Heiki Kanojo, I: Wish You Were Here Recommendations
They both have a teenage girl being turned into a weapon to save people, they both have romance and drama, but Zaion contain less romance.
Both series features science fiction themes in an apocalyptic world involving the main protagonist and a young girl.
Both series contain the theme of the young girl being weapons of destruction, a theme that is tragic and emotional.
Both series contains action, drama, tragedy, romance, and a mystery like atmosphere that gives off a similar feeling.
Both stories focus on a young male protagonist, trying to save a love interest who is being used by the government as a deadly human weapon.
Saishuu (You'd look up Saikano in English for whatever reason.) is much more deep in character, in emotion, and in romance; where Zaion is more an action-based story of two characters who chance to meet in the worst possible situations.