Seiya Ryuuguuin is depicted as a tall and well-built young man with a muscular physique and sharp features. He has black spiky hair that is neatly styled, often falling over the right side of his face and partially obscuring his right eye. In the manga adaptation, his hair appears more tousled and swept back, giving him a slightly more rugged look. His eyes are black and typically maintain a composed, contemplative expression though they shift to intense or irritated when provoked. His striking appearance often garners admiration from nearly everyone he encounters, including deities and adversaries alike.

Seiya’s standard attire includes a fitted white undershirt beneath a black jacket and matching black pants, complemented by armor pieces that protect his chest, shoulders, and joints. Hidden beneath his outerwear are oversized enchanted bracelets that provide various resistances and stat enhancements. Though their size adds some physical burden, Seiya considers the protective benefits well worth the weight.

In the manga version, his outfit remains largely the same but features subtle differences, such as the inclusion of a utility-style belt and more contemporary-looking footwear, deviating slightly from the anime’s more medieval aesthetic. Upon his initial summoning to the world of Gaeabrande, Seiya briefly appears in modern, casual clothing before adopting his battle-ready gear.

Seiya Ryuuguuin is best known for his extreme and often exaggerated cautiousness, which defines nearly every aspect of his behavior. He adamantly refuses to engage in combat unless he is absolutely certain of victory, and will relentlessly attack already defeated enemies to ensure they cannot regenerate or resurrect. His lack of trust extends to both allies and strangers alike, as he believes anyone could be an enemy in disguise or a carrier of a deadly disease.

While his paranoia can be frustrating to those around him, Seiya’s caution is rooted in deep diligence and discipline. He is exceptionally committed to preparation, often spending days grinding levels or training in isolation to guarantee that he is at peak performance for any potential threat. His meticulous approach to combat makes him incredibly effective, though also emotionally distant.

Seiya is notably blunt and socially unaware, often disregarding the feelings or boundaries of others. He lacks a basic understanding of social norms, performing actions such as dumping holy water on strangers or picking hairs off Ristarte while she sleeps completely oblivious to why these acts are considered inappropriate or invasive. He also fails to recognize romantic cues, remaining indifferent to the affections directed at him.

Beneath his cold exterior, however, Seiya is deeply protective of those he cares about. Though he often distances himself from others emotionally, this behavior is a defense mechanism born from a fear of loss. His determination to protect his companions even at the cost of his own emotional well-being reveals a selfless side that he rarely expresses in words.

Seiya shows no apparent interest in pleasure or leisure, displaying apathy toward food, celebrations, and even romantic or sexual encounters. His focus remains unwaveringly fixed on the mission at hand and the annihilation of any threat to the world he’s sworn to save.

Despite his arrogance, lack of tact, and emotionally detached behavior, Seiya is ultimately a kind-hearted individual whose every action is driven by a desire to prevent harm and ensure the safety of those around him.

Long before his summoning to save the world of Gaeabrande, Seiya Ryuuguuin was previously summoned by the goddess Ariadoa to act as the hero of the B-ranked world of Ixphoria, an earlier divine mission that would ultimately shape the foundation of his cautious personality.

In stark contrast to his current persona, Seiya was once reckless, overconfident, and dismissive of training. Despite warnings from his companions Princess Tiana and Colt, a loyal swordsman, he often rushed into battle with minimal preparation. Nevertheless, his natural strength and combat instincts led to repeated victories, allowing the party to overcome every obstacle thrown at them. In time, they succeeded in defeating the Demon Lord of Ixphoria after an arduous and hard-fought battle.

However, just as the group was celebrating their triumph, tragedy struck. The Demon Lord, still alive due to possessing a second hidden life, launched a surprise attack and fatally wounded Colt by impaling him through the chest with his tongue. The demon then swallowed Ariadoa to restore its power and revealed that it had merely feigned defeat. Realizing the overwhelming danger, Seiya urged Tiana to flee but the Demon Lord targeted her first, revealing that she was pregnant with Seiya’s child. In an act of cruelty, the Demon Lord murdered both Tiana and the unborn child in front of Seiya before killing him as well.

Following his death, Seiya was reincarnated and summoned again this time by Ristarte to save Gaeabrande. Although his memories of Ixphoria were sealed, the trauma and grief from his past failure remained engraved in his soul, manifesting as the overwhelming caution and emotional detachment that now define him.

Seiya Ryuuguuin’s greatest asset is his extreme caution, which allows him to meticulously plan for nearly any situation. His tendency to imagine worst-case scenarios makes him highly unpredictable in combat, as he prepares for possibilities that most would overlook. This hyper-preparedness, while often played for comedic effect, proves to be one of his most valuable traits in life-or-death encounters.

Seiya is also an exceptionally fast learner with a natural aptitude for mastering new combat techniques. Combined with his relentless training regimen, he pushes his physical capabilities to peak human condition. He possesses immense speed, strength, endurance, and is highly skilled in swordsmanship, often outmatching seasoned warriors through both technique and strategy.

Despite his overwhelming power, Seiya is not invincible. He can still be injured, particularly by enemies with superior stats or unexpected abilities. However, his cautious approach often includes countermeasures and backup plans to mitigate such disadvantages.

Among his learned techniques, one of the most dangerous is Valhalla Gate, a powerful summoning spell capable of destroying nearly any enemy. The ability, however, comes with a fatal drawback: it sacrifices the user’s life upon activation, making it a last-resort measure.

Additionally, Seiya possesses a passive experience boost ability, allowing him to level up at an accelerated rate compared to other heroes, enabling him to quickly outpace his enemies through constant training.

The given name Seiya (聖哉) is composed of the kanji 聖 (sei), meaning “holy” or “sacred,” and 哉 (ya), which is an archaic exclamatory particle often used for emphasis, roughly translating to “what,” “how,” or serving as a question mark. Together, the name can be interpreted as “O holy one” or “sacred man,” which aligns ironically with Seiya’s status as a summoned hero, despite his cold and pragmatic personality.

His surname, Ryuuguuin (竜宮院), combines 竜宮 (ryūgū), meaning “Dragon Palace”, a mythical undersea palace from Japanese folklore ruled by the Dragon King and 院 (in), which typically means “institution” or “temple,” and can also refer to places like hospitals or academies. Thus, the name may loosely translate to “Temple of the Dragon Palace” or “House of the Dragon King,” evoking a sense of grandeur and mysticism.