I’d like to see this as an anime (confession: I don’t normally have the patience or cash for manga). It just sounds amusingly weird. Mysterious Girlfriend X:

Mysterious Girlfriend X is the story of high school student Tsubaki Akira … and his classmate, a strange scissors-wielding girl named Urabe Mikoto…, who begin an unusual romance after he becomes addicted to her saliva.

….

[Three are] subtle hints that she might be an alien (a UFO key chain, UFO pajamas, UFO pen, etc).