At the moment, she is one of the main characters in the game. Therefore, you can only be friends with her, no options.
If you don't like the very specific "female friendship" in all its forms, then you'd better wait for the final version of the game. There's a chance that there will be some variability.
And if you like relationships between guys and girls, then for now this topic is very bad here. The maximum: erotic clothes for guys and girls.
Well, Nel is a key character in this game and she plays an important role in Celica's life.