Marvelous had fans vote to select the character for the cover honors of Senran Kagura Burst's bonus "Girls' Secret File" visual book. Fans voted in large part for one of the new ninja girls, Mirai.
Here's the resulting cover:
I believe the little sign on Mirai's legs is asking people to pick her up and take her home, like one might take home a stray cat. The concept for the cover is, in fact, "cute stray cat."
Why Mirai has been placed in a box is something we won't known until the police have completed their questioning of the game's artist.