I am confused now.

Is this supposed to be a event?

On Nut it is advertised as such. Ingame it is not.

The banner says "Girl on Girl action". As if this was a time limited event. It even has a countdown.

Ingame it has no countdown. Previous events had one. It just says "New". And all of it looks and feels like a new game mechanic. A sort of meta trophy or extended trophy - you get diamonds and items after the 20+ diamonds for completing a girl. Now you have to win challenges with her. Was there ever an event where you could gain diamonds, instead of spending them?

Furthermore, in the memory tab it is sorted in front of all the other events and after The day we met. It is named Getting clean there.

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What I am curious, how will the game decide, who your best girl is and does it do so with the sum of all base stats or only if all base stats are higher.