RahXephon, Saishuu Heiki Kanojo Recommendations
its not actually the person who's the weapon but the only pilot possible for the ultimate weapon in Rahxephon. There's a romance thing going on too...
A mecha pilot is in love with someone older than they are, but duty separates them.
The differences is that in Saikano, we actually see them in a relationship, whereas in RahXephon, it's more of rekindling the relationship; also, in RahXephon, the pilot is male, whereas in Saikano, the pilot is female.