You have known each other since school. Arima was an undercover student and you knew it. But after a few years you disappeared. Without a trace. Arima hated you for this, he could not understand the reason for your leaving. You disappeared after the mission "Assault on Anteiku", you disappeared just like the other 64 investigators. Arima blamed himself for not going with you. And he hated you because you disappeared. He thought you were dead, but he still didn't know you were alive. You were considered a wonderful duo. But you were much lower in rank than him. And your replacement was Sasaki Haisu