In this case, this is already a gaming convention. In many situations, the player cannot do otherwise, because continuing the gameplay will become impossible.
But as for this mechanic, there is still an important difference from classical relationships: if you communicate with your friends or you even make love with them, this is an interaction of bodies and personalities.
As for friendship with ghosts, here something else is already obtained. Something like collective ownership. But if the concept of property in the 20th century usually meant fields and cows, then in Celica’s case, it was her body.
(It seems to me that for this site we are considering too complex topics :-) )