This is an event-limited version) of Momoko that debuted some weeks ago, said event being Halloween-themed. Her transformation sequence can be seen here (which I think looks very cool).
For "flag reason appears to be wrong" - if you pick any file in this Magireco file mirror that ends in c.png, you will find that there is no frame on any of the images (with an exception of 40285, which is an error). The image can be viewed with a frame either in-game or on that site's news mirror (different announcement since this unit was released too long ago, but the mechanism stays the same). The frame can also easily be found here on the same file mirror. So this either is a screenshot, or a third-party edit. The original artwork does not have a frame embedded into the image.