Kotori Hidaka

A girl who made a promise to go to space together with her friends Nozomi and Hayato. After Nozomi's death, Kotori was told he had "gone to space", and has continued to convince herself it was true ever since.

From Giko's Girl Dictionary:

A girl who longs for the sky.

Mankind has always dreamed of being able to fly. Her mental instability might have left her more vulnerable to the temptation of the heavens than most.

I'm sure she'll prove to be valuable for further research into Maidens in Love, one of the most difficult subjects I have ever come across.