Jun Mochizuki has shown a deep interest in symbolism since at least Pandora Hearts, hinting at plot events and character motivations in covers and illustrations well before they were revealed in the story. This has continued into her latest work—Victorian flower symbolism abounds with the addition of Dutch vanitas painting symbolism.
Every volume cover has a picture frame containing a memory or idea important to the character on the cover. Other times the frame’s contents have been current or future plot events. (As an aside, the entire Case Study of Vanitas can be considered memories; Noé is writing the story from the future.) The frames are typically ornate and contain both vanitas and flower symbolism. So far, volume 1’s cover is the densest example of this, and perhaps will be the most fruitful to return to in years to come.