Develop relationships with the characters and Nel will react to it.
It makes sense to go to joint duels only with good friends.
Relationships can roughly include leveling up Celica’s friends in battles. It is important that in addition to the filled scale on the left side of the game screen, they also have enough experience in jointly participating in duels with monsters.
The more the body trusts a particular spirit, the easier it is for that spirit to gain control.
It is better not to continue this thread of questions, since due to the number of new questions, the comments in this thread are already difficult to notice.
Therefore, it is better to ask a new question if necessary.