True Worth of a Friend (友情の資格 Yūjō no Shikaku) is the fourteenth episode of Phi Brain: Kami no Puzzle Season One and the fourteenth episode of the anime series overall.
As Kaito remains unconscious in hospital, Cubic is invited by Rook to take on a puzzle and accepts it, wanting to prove his friendship to Kaito. He comes to an abandoned military base where a different POG Giver gives Cubic a math puzzle to solve, where his failure will result in him being blown up by a tank. Meanwhile, Kaito regains consciousness and explains his situation with Rook to the others. Cubic is given the chance to escape, but he remains determined and manages to solve the puzzle at the last second. The Giver reveals that Rook made the puzzle when he was five, which makes Cubic feel unworthy of being Kaito's friend in comparison.