She is integral to the part after Persona 5 regular ends, i.e. the "DLC" story, along with another new character.
During the old Persona 5 story, she's only there superficially and you can't even add her to your party. And when her story starts, you can take her out of the party for 95% of it, as I have done.
I found her presence inoffensive and the new antagonist is pretty interesting, so he makes the experience of the Royal content.