What is Mathias's relationship with Tiddy?

Tiddy is an officer with the Population Police. He is very close to the commander. When he is caught up in a gunfight between Population Police and rebels, Tiddy is the only officer who manages to escape and he does so with Matthias's help. Tiddy returns to the base, promising Matthias he'll be accepted as a Population Police officer-in-training. At the base, Tiddy goes directly to the commander with the story of the dozens of rebels who had him pinned down until Matthias stepped in and saved him. The story is greatly exaggerated and Matthias calls him on it, asking why he'd told that story. Tiddy responds that Matthias can't be sure there weren't that many rebels in the opposing force. Tiddy is self-confident to the point of arrogance but is so well liked that people overlook his arrogant attitude. Tiddy's introduction of Matthias to the commander puts Matthias in a position to be accepted by the commander after Tiddy's death. Tiddy dies of poisoning and Matthias is struck by his death but not to the degree others believe. Over the coming weeks, Matthias often blames his grief over Tiddy's death for his brooding attitude though he is actually grieving over Percy and Alia, who he believes is dead at the hands of the Population Police.