A common trope in harem romantic comedies is to introduce a new potential love interest to the main character when the series is getting stale. I think it's quite the achievement for Chitose Is In The Ramune Bottle to not have introduced a new heroine in the previous 6 volumes.
I wasn't the fan of the conclusion of the sixth the volume and how the characters decided to embrace the stagnation of their romantic relationships. Having a new character enter the series to shake up the relationships of the characters was the obvious course of action for the writer but sometimes the obvious choice can be rather boring because of how predictable it is.
Kureha at first seems like a very generic character. A classic kouhai late heroine type character. Bubbly, innocent, kind, and looks up to her senpai. A character who only exists to make pre-existing heroines jealous of her and spark up interactions within the story. Kureha is all that but surprisingly done in a clever way.
I jokingly predicted to myself the twist of the book but I was still surprised by the twist that Kureha was actually super calculating in her actions and was just pretending to be an innocent kouhai who was getting close to the main character for completely wholesome reasons.
Having a character deliberately pretend to be the exact thing the plot needed in a meta sense instead of just having a basic kouhai love interest adds a layer of tension to the whole thing and it makes for a great twist to a classic trope in romcoms.