Mueller, call sign Blue Angel, was an OZ Lieutenant and Aries pilot who was trained by Noin at Lake Victoria Academy and then joined OZ. He was a member of the Number 33 Independent Troops of Northeast Africa, Somalia front. Along with Alex, he was hunting down former Alliance members and attacking them in the name of Treize Khushrenada. He was killed by Zechs after violating the terms of surrender during an attack on the Mogadishu Fortress. Mueller was significantly more hot-headed than his partner Alex.