Ernst Zimmerman rose from imperial general to interim president of the Federal Republic of Giad, leading revolutionary forces to overthrow the oppressive Giadian Empire. His disillusionment with imperial policies fueled the rebellion, establishing him as the fledgling nation's first president. Despite his Jet serf-class heritage, he commands support from the powerful Onyx families.

Physically, Ernst presents as a middle-aged man bearing the ink-black hair and eyes of his Jet lineage, now touched by gray. His slender frame carries a scholarly air, accentuated by silver-rimmed glasses and a straight mustache. He adopts the standard Federacy panzer jacket uniform for military engagements like the Morpho Elimination, distinguished by unique rank slides marking his role as commander-in-chief.

Ernst embodies a duality of character. Publicly, he is a staunch idealist, fiercely protective of universal justice and equality. This fervor earned him comparison to a "fire-breathing dragon," willing to destroy even humanity should it abandon compassion, especially towards children. Privately, he assumes a laid-back, paternal role, creating a new household to compensate for the loss of his original family before the revolution. He exhibits deep protectiveness towards those under his care, viewing them as surrogate children.

His personal history includes the pre-revolution loss of his wife and biological family. During the Siege of Rosenfort, he faked the death of the last Giadian royal, Augusta Frederica Adel-Adler, adopting her as Frederica Rosenfort. Later, he extended guardianship to the five survivors of the Republic of San Magnolia's Spearhead Squadron, including Shinei Nouzen, offering them sanctuary and familial structure. His relationship with Frederica involves mutual respect tempered by discipline, such as assigning arithmetic drills after unauthorized battlefield excursions.

Ernst's leadership during the Morpho Elimination showcased his convictions. He mobilized the Western Theatre Forces with a rallying speech framing the battle as a defense of humanity's future. When staff suggested abandoning soldiers in Legion territory, he threatened lethal consequences, declaring unethical survival tactics invalidated humanity's right to exist. This commitment positions him as a foil to figures like Jérôme Karlstahl, another disillusioned officer who chose resignation over revolution.

Within his household, Teresa, a maid resembling his deceased wife, manages domestic affairs. Ernst clarifies he feels no romantic attraction toward her, emphasizing his focus on familial bonds. His interactions with the Spearhead Squadron highlight his desire to shield them from further trauma, urging them to "come back alive" and providing a stable home. After the Republic's fall, he facilitated Giad's assistance to San Magnolian survivors like Vladilena Milizé, indirectly renewing Shinei Nouzen's sense of purpose.