This is for lovers of the body ownership fetish; the level of influence on the character’s body is indicated.
If this fetish doesn't interest you, just ignore it.
Does it affect gameplay?
I don't have this fetish but I still want to play along. I enjoy the social aspect of it all. If people invade my space too much I want to stick up for myself, but sometimes the game expects me to be overly passive toward people who haven't shown me they can be trusted and I'm not always able to continue otherwise.
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 :-) )
I believe this part of the game is just a little too much and there isn't an alternative. You have to give up your body, or be submissive to continue the gameplay and storyline.
There is supposed to be a reward when you reach max relationship, but the crap you have to put up with weakens the reward(s) to the point where they are useless or meaningless.