Gin Gagamaru is a tall, eighteen-year-old high school student from Wakayama Prefecture, standing at 191 centimeters (approximately 6 feet 3 inches) with a lean and lengthy build. He has sharp black eyes and long black hair that is typically tied in a bun, which he later dyes a silver color. A notable feature is the series of scars on his left ear, which he received from an encounter with a bear.

Often called a wild child by those around him, Gagamaru has a simple, pure, and deeply instinctual personality. He lives by a personal philosophy of going with the flow of nature and believes in accepting things as they are. He prefers the wilderness over modern technology, struggles to understand the feelings of other people, and is known for eating food with his bare hands. His understanding of human emotions and drama comes almost entirely from reading manga. He is honest, straightforward, and generally nonchalant, accepting changes to his position or role on the team without protest. Despite his unconventional behavior, he is viewed by his teammates as a reliable and adaptable player.

Gagamaru started playing soccer relatively late, only joining his junior high school's club at the age of twelve. Before entering the Blue Lock project, he was a member of the soccer club at Yasei High School. His unusual upbringing in the wilderness, where he spent time in the mountains meditating in bamboo groves and observing animals, gifted him with a naturally powerful and agile physique.

Entering Blue Lock initially ranked 296th, Gagamaru began the program as a forward for Team Z. In the first selection, he specialized in close-quarter plays and goal-poaching, using his body to score goals from difficult angles. During a match against Team V, his quick reaction to a free kick led to a crucial save and a subsequent counter-attack that secured Team Z's victory. He successfully advanced through the second selection, ranking 88th and forming a team with Jingo Raichi and Junichi Wanima. In the third selection, he volunteered for Team C, where he showcased his defensive skills with acrobatic interceptions.

Through these trials, it became clear that Gagamaru possessed the ideal physical build, natural reflexes, and skills for a goalkeeper. For the major match against the Japan U-20 team, he was selected as the starting goalkeeper. In this role, he made several critical saves against powerful shots from elite players using creative and acrobatic techniques.

His development continued in the Neo Egoist League, where he chose to play for the German team Bastard München. Despite not specifically training for the position, his high aptitude in practice quickly earned him the role of the main goalkeeper. He has made notable saves against powerful strikes from players like Reo Mikage and Shoei Baro, significantly increasing his market value with each performance. His key relationships are primarily with his former teammates on Team Z, especially Yoichi Isagi, with whom he shared the field during the first selection.

Gagamaru's most notable abilities stem from his exceptional physical gifts. He possesses a spring-like body, with naturally long limbs and excellent body control that allows him to adapt to nearly any play. This is complemented by his explosive reaction speed, enabling him to make last-second saves without wasted movement. He has superb vision rated at 6.0 and strong hearing. His flexible and acrobatic style allows him to perform unorthodox blocks, including a scorpion kick technique, where he propels his body upward and flicks his heels back to make contact with the ball. He has been recognized as having the highest goalkeeper aptitude rating within the Blue Lock project.