Muller is a secretary in the Marleyan military. When most of the continental mainland of Marley is annihilated by the Rumbling, Secretary Muller rallies Marley's remaining forces to Fort Salta in a desperate final effort to stop the Founding Titan's advance.
Muller is an older man with short light hair styled in an undercut and a thick beard running the length of his lower jaw. He also has noticeably thick eyebrows. He has only been seen wearing the standard Marleyan uniform.
Muller seems to be a rather introspective man, capable of recognizing the faults of Marley. He is able to recognize Marley's hand in radicalizing Eren Yeager into beginning the Rumbling, and makes the claim that he will change his ways if he is given the chance to. However, it has also been shown that he is a rather fearful person, as he orders the airships to begin bombing immediately upon a slight movement of the Attack Titan, despite the airships not yet being at the right altitude, while also being visibly frightened.
Appearance: Muller is an older man with short light hair styled in an undercut and a thick beard running the length of his lower jaw. He also has noticeably thick eyebrows.
Personality: Muller seems to be a rather introspective man, capable of recognizing the faults of Marley. He is able to recognize Marley's hand in radicalizing Eren Yeager into beginning the Rumbling, and makes the claim that he will change his ways if he is given the chance to.
Muller in the story: After Marley is ravaged by the Rumbling, Muller leads a contingent of Marleyan soldiers in a last-ditch effort to stop Eren Yeager from Fort Salta. As the base's airships deploy to bomb Eren's Founding Titan, Muller reminds his soldiers of the responsibility they hold. However, he also reminds them that Marley's hatred for Eldians is what has brought this attack on their nation and promises to change his ways if he is ever given the chance.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.