Sir Thursday (The Keys to the Kingdom)

In this fourth installment of the series, Arthur and Leaf manage to survive their ordeal in the Border Sea and attempt to return to their normal lives on Earth. However, upon arrival, Arthur discovers that a shape-shifting dopplegänger has stolen his identity and completely taken over his life, locking him out of his own world. Before he can fix this, Arthur is forcefully conscripted into the “Glorious Army” by the rigid and militaristic Sir Thursday. While Leaf remains on Earth attempting to stop the imposter, Arthur must brave grueling military training and find a way to secure the fourth part of the Will to regain his freedom.