(OOC; I've changed part of Shiko's backstory, and I'm going to start working my new plot into this. Basically there was another demon weapon working with Medusa for a short time called the Demon Executioner, but their actions were blamed on Crona because the Executioner was rarely seen and preferred to strike from the shadows. As you've probably guessed by now, that person was Shiko. Crona would have never met them in person, nor would they be aware of their identity, but was aware of their existence.)
(Since this story should be taking place shortly after the Trial Class arc, it's safe to assume that the betrayal of Marie-sensei has already happened, and Shiko would be aware of what Crona has done even if no one else is. Shiko was allied with Medusa until right before the events of this rp.)