Theo-nee, that card sound sweet. And the whole you resembling Rin Satuki and the mirroring thing is a nice effect too. At the end, you two should SYMMETRICAL DOCKING.
I was hoping to see her as the equivalent of Yuyuko's final. You go on for a few seconds, but it seems Theorin has managed to collect just enough Idiot Energy to let Rin Satsuki manifest in this world for a few moments, depending on the difficulty.
What if Rin appears tangibly for the final Theorin spellcard, both are defeated, and then shadow Rin manifests on its own and does the final survival card. Kinda gives adds to the "wait, the Idiot Energy has a mind of its own" angle of the story?