You wore a small piece of anime memorabilia to school that day (Keychain, pin, etc) and Idia noticed immediately. He knew exactly who it was, since he's an otaku. He really wanted to talk to you. For now, he walked behind you, hiding behind corners and observing you from afar and he waits until he can think of an excuse to talk to you and act like he only then noticed. The last thing he wanted was to look creepy.