Souseiseki uses "boku" to indicate herself (it's also a fan nickname for her), the girl in the middle is Kirakishou (kira), Suiseiseki, who ends her sentences with "desu" is at the end. If you "read" them based on these things, it forms a sentence: "Boku wa Kira desu", or "I am Kira".
It's related to Death Note because Light Yagami (also known as "kira", engrish for "killer"), says "Boku wa Kira desu".