 If you liked
Blue Gender
|
 ...then you might like
I: Wish You Were Here
|
Similar feeling, the earth have been infected with monsters in Blue Gender and a virus in Zaion which turn the user into a monster. The two main characters in both anime have a similar relationship too as they are mostly treated as tool.
I think Zaion have better romance but Blue Gender is overall better because the serie is longer and have more solid content.