User blog comment:TimeSoul/Detentionaire: Season 1 finale/@comment-99.237.208.162-20120331212847

I'm guessing it's Mrs Ping.

Rad circles sounds math related, which is related to Lee's mom.

The knock knock at the end of episode 12 sorta seems like its foreshadowing who it is and plus, Lee's mom surely has connections with the secret underground tunnel (which is shown throughout the show).