Eurus is a Guardian, an ancient being who serves as a protector of the portal between the worlds of Quarton and Earth. His actions are driven by a strong belief that a fellow Guardian named Boreas has disrupted the natural balance of Tenkai Energy, a fundamental force in the universe. Eurus's primary motivation is to correct this perceived imbalance, a goal he pursues with a willingness to employ morally ambiguous methods. He claims to have assisted the Tenkai Knights in their acquisition of a new power and has also been known to aid the antagonists Beni and Gen in their search for the powerful Black Key. His methods appear to be strategic and independent, as he expresses a desire to help the other Guardians after he deems that the villain Vilius has gone too far in his destructive actions. This indicates that while his original goal is focused on cosmic balance, he is also capable of reassessing his alliances when a greater threat emerges.

Within the story's events, Eurus initially operates as a complex figure whose help to both sides creates uncertainty about his true loyalties. His role shifts dramatically when he is captured by Vilius, becoming a component in the villain's new scheme. Prior to his capture, it is revealed that he gave Gen the core brick that would allow the creation of the corrupted knight Dromus, and he was also the one who suggested the idea of awakening an evil dragon. These actions portray him as a catalyst for significant conflict, driven by his personal belief system rather than a simple allegiance to good or evil. His relationships with the other Guardians, including Boreas, Zephyrus, and Notus, are central to his background, though these dynamics are largely defined by his opposition to Boreas’s perceived mismanagement of Tenkai Energy. He also has direct dealings with the Tenkai Knights—Bravenwolf, Tributon, Valorn, and Lydendor—as well as with corrupted knights like Dromus and Venetta.

In terms of his development, Eurus begins as an enigmatic helper whose assistance comes with its own agenda. His eventual claim that he wishes to help the other Guardians stop Vilius suggests a possible shift in priorities, though his capture prevents the full realization of this change. His abilities are considerable, marking him as a master of Tenkai Energy. In his original form, he possesses the power of flight, enhanced speed, and the skill of deflection. He is also a proficient marksman. When he assumes his Dark Tempest Mode, he retains all of these abilities, indicating that this transformation augments his power rather than fundamentally altering his skill set. The name Eurus is derived from the Greek god of the east wind, a figure associated with rain, warmth, and unluckiness, which is reflected in his red color scheme.