Kazuyoshi Igarashi, often called by the nickname Igaiga, is a central figure whose life story is defined by a dramatic fall from grace and a search for redemption. He is a former professional golfer who once had a successful career, a wife, and a son. However, his life took a sharp turn following a scandal involving illegal gambling. He became involved in high-stakes golf matches with gangsters and accepted a significant amount of money, which led to the loss of his professional qualification and, consequently, the breakdown of his family and his divorce. After losing everything, he struggled to find stable work and eventually moved to the remote Hinoshima island in the Tokara archipelago after seeing a job listing, relocating even before his official interview.

In terms of personality, Igarashi is logical, methodical, and careful, especially when it comes to the sport of golf. He is a rational thinker with a strong analytical mind, which makes him a surprisingly effective teacher, even though he generally avoided coaching amateurs in the past. However, he is also a man deeply affected by regret and shame over his past decisions. He can be blunt and has made mistakes with the people he cares about, most notably his son, which has strained their relationship. He carries the weight of his failures but also demonstrates a capacity for self-reflection and a desire to become a better person. On the island, he initially presents himself as a weary and contemplative figure, but he becomes increasingly invested in his new life and the people around him.

Igarashi's primary motivation shifts over the course of the story. Initially, he is simply trying to escape his past and find any stable job to survive. However, after meeting Tonbo Ōi, the island's only middle school student, he becomes captivated by her raw, unorthodox, and instinctive talent for golf, which she developed using only a single 3-iron club. Her free and powerful style, honed by the island's unique environment, rekindles his passion for the sport. His motivation transforms from mere survival to a genuine desire to mentor Tonbo and help her unique talents succeed in competitive golf, seeing in her a second chance to make things right. This role as her coach also provides him with an opportunity to confront his past failures as a father and mentor, effectively becoming a paternal figure to Tonbo.

In his role as the story's deuteragonist, Igarashi serves as Tonbo's coach and guide. He uses his deep knowledge of professional golf to help shape her raw talent into a competitive force, teaching her strategy, technique, and the nuances of the game without suppressing her natural creativity. His own journey of recovery and self-forgiveness is intertwined with Tonbo's growth as a golfer. He is the bridge between her isolated, instinct-driven world on Hinoshima and the structured, high-pressure world of professional tournaments. Igarashi is also the character through which the series explores themes of regret, fatherhood, and finding new purpose.

His key relationships are central to his character development. His relationship with Tonbo Ōi is the most significant. He moves from being a curious outsider to her dedicated mentor and a surrogate father figure, a role he cherishes as it allows him to correct the failures of his past. This is directly contrasted with his strained relationship with his own son. When his son became interested in golf, Igarashi failed to support him, offering harsh criticism and dismissive words instead of encouragement, which led to their estrangement. His relationship with his ex-wife ended in divorce following the loss of his pro status. Through teaching Tonbo, Igarashi seeks to redeem himself and prove that he can be the supportive and effective mentor he failed to be for his own child.

Throughout his journey, Igarashi demonstrates notable abilities. He is a highly skilled golfer, even after losing his professional standing, and possesses a sharp, logical mind that is exceptional for analyzing swings and teaching. He has a natural talent for explaining complex concepts in a way that others can understand, a gift he initially shied away from using. His key ability is not just playing golf, but seeing and fostering potential in others, especially unorthodox talents like Tonbo's. He is able to break down her instinctive movements and translate them into teachable techniques, helping her progress while ensuring she stays true to her unique style.

Igarashi's development is the core of his character. He begins as a man broken by his past mistakes, full of cynicism and regret, who has fled society. Through his life on Hinoshima and his commitment to coaching Tonbo, he gradually re-engages with the world. He rediscovers his sense of purpose and self-worth, moving from a passive state of hiding to an active role as a mentor and, eventually, a father figure. He is a character who actively seeks to atone for his past by ensuring he does not repeat his mistakes with a new generation, finding in his role as Igaiga a new and more meaningful identity than the one he lost as a professional golfer.