Dave Quagmire, known in his original appearance as Dvalin, is a talented and driven footballer who first emerges as a formidable opponent during the Aliea Academy arc. His real name is Osamu Saginuma, and he was one of the children who grew up in the Ohisama En orphanage. He was selected by the project's mastermind to be enhanced by the energy of the Aliea Meteorite, transforming him into a superhuman player as part of a plan to create the ultimate soccer team. As a ninth-grade student, he is given the alien alias Dvalin and appointed as the captain of Epsilon, one of the elite teams within the Aliea Academy's hierarchy.

In terms of personality, Dave Quagmire is defined by his meticulous nature and immense willpower. He is a perfectionist who finds satisfaction only when events unfold exactly according to his plans, and he becomes visibly frustrated or loses his composure when things go wrong. Despite his dark and serious demeanor as an antagonist, his personality reveals a deep-seated passion for soccer that seems almost limitless. He is genuinely excited by displays of strong will and perseverance, and he develops a respect for opposing players who demonstrate these qualities, particularly the main protagonist. This respect shows that even while serving as a villain, his love for the game is authentic. Later in the series, his character matures significantly. He becomes responsible, understanding, and a good leader, taking on a role that is described as being almost like an adult compared to his peers. He is also depicted as being somewhat poor at inspiring others through writing, as a letter he composes ends up being overly long and depressing rather than motivational.

His primary motivation throughout the story evolves. Initially, as the captain of Epsilon, his drive is to fulfill his mission for the Aliea Academy, which involves defeating other teams to find strong players and ultimately gain more power. He is presented as extremely determined and even greedy for more strength and opportunities, such as a chance to play on an international stage. However, after the Aliea incident is resolved, his motivation shifts to proving his worth on a legitimate global level. Feeling hurt and rejected at not being chosen for the main national team, he channels that drive into forming Neo Japan, a rival team composed of players who were also overlooked, with the specific goal of defeating Inazuma Japan to earn a spot in the Football Frontier International tournament.

In the story, Dave Quagmire serves as a major recurring rival. He is first introduced as the captain and goalkeeper for Epsilon, a team sent to take over after the failure of the previous Aliea squad, Gemini Storm. His initial match against the Raimon team is one-sided, but he makes the surprising decision to forfeit, allowing his opponents to train and become stronger so that he can face them at their best in a serious rematch. This early action foreshadows his genuine love for competitive soccer. He later returns with an upgraded team, Epsilon Plus, where his own power is further enhanced and it is revealed that his original position is actually a striker, not a goalkeeper. After the Aliea Academy arc, he takes on the role of captain and a midfielder for Neo Japan, a team that acts as a final obstacle for the main characters before they can represent Japan. In later series set in an alternate timeline, his role shifts once more, appearing as a co-coach for Mount Olympus and also as a goalkeeper for Inazuma Japan in the Orion arc.

Key relationships help define his journey. His most significant relationship is with the main protagonist and captain of Raimon. Their matches are the catalysts for his change; the protagonist's unwavering willpower and ability to grow during a match are what rekindles Quagmire's pure excitement for soccer, breaking through the artificial influence of the Aliea meteorite. Following the Aliea incident, he works closely with Lina Schiller, who coaches Neo Japan. Under her guidance, he continues to train with immense dedication, even returning to practice in the goal alongside her after his team's defeat.

His development is a clear arc from a powered-up antagonist to a respected rival and finally to an ally. He begins as a cold and calculating tool for a larger conspiracy, but his genuine passion for the sport re-emerges through competition. He openly acknowledges his defeats and uses them as fuel to improve, demonstrating a capacity for growth and sportsmanship. By the time of the Inazuma Eleven GO series, he has matured into a coach and assistant, having left behind his villainous past and abandoned the shadowy organization he once worked for.

Notable abilities make him a versatile and powerful player. His main position is as a goalkeeper, but he is also highly skilled as a striker and midfielder, making him a valuable asset in multiple roles. He is associated with the fire element, though he showcases the wind element when playing as a striker. His signature defensive hissatsu technique is Drill Smasher, a powerful move where he creates a giant metal drill to stop the ball. His most famous offensive technique is the shoot hissatsu Gungnir, where he kicks the ball through a portal that turns it into a celestial spear, a technique so powerful it can create a large, lasting whirlpool. He also possesses the Wormhole, another formidable goalkeeping technique that sucks shots into a portal. Both his physical and mental attributes are high, with his game statistics showing exceptional control, technique, and intelligence, reflecting his role as a field general and captain.