Now all of you It's already at 200. But the relationship is at 50, how does it increase?
If you are talking about the relationship of most girls being stuck at 50 there is gift items you can give them https://renryuu.miraheze.org/wiki/Items scroll down to the gift section.
https://renryuu.miraheze.org/wiki/Raids down near the bottom shows you what items are needed to make what item. It is random each time.