Cyrus objected to Caulder having Penny attack Brenner's Wolves, as the battalion had done nothing to provoke assaulting them. However, seeking to gather more information, Caulder ignored Cyrus' protests and had Penny go forward with the attack anyway.
Following the fall of the New Rubinelle Army, Cyrus appeared alongside his siblings as their father noted how interesting it was that the 12th Battalion triumphed over the NRA due to superior tactics and not more advanced weaponry. He then ordered his children to engage the 12th Battalion in order to obtain more data.
However, fed up with his father's experiments, Cyrus defected over to the 12th Battalion and helped Will get aboard the Great Owl by hacking its computer system so he could rescue Isabella, who was in fact another clone created by Caulder originally named "Lutaria", as explained by Cyrus.
Foreseeing Cyrus' betrayal, Caulder ordered Penny to crash the Great Owl, but she was talked out of it by Isabella. Caulder then tracked Cyrus down and poisoned him in retaliation for his betrayal. However, Caulder had the poison kill him slowly in order to torture him, granting Cyrus enough time to tell Will and the others the location of Caulder's base The Nest before he died.
Cyrus' unexpected heroism was later noted by Dr. Morris, who stated that they would not have found a cure to the Creeper virus without him.
- For some reason, he addresses the main character Will by his full first name William.
Advance Wars: Days of Ruin