Oh, you already have a character in mind? *ponders*
I was going to suggest Tea... I knew she's spent a few anxious moments by Seto's side in your fics (and probably will in mine...), but never the reverse...? Maybe Tea was due back from her recital... but never came back? And she hasn't spoken to Yugi or Joey or anyone? Maybe she had a fever and went to recital even though she had been told to stay home, but never even made it to recital? So she's not there, and she's not at home... but if you already have a character in mind, you could still have them under the weather and still intending to go somewhere, but not arriving, therefore driving everyone crazy?
I was going to suggest Riku, too... Maybe something happened on Destiny Islands, and Riku vanishes again, leaving Sora and Kairi worried that the darkness is back and is behind Riku's disappearance? But, again, if you already have a character in mind, a general arrival of dark creatures amidst disappearance of said character would also work...
And yay for Seph's friendship with Cloud and coaching him to be his best!