Grandpa Yukawa is the elderly patriarch of the Yukawa family and the keeper of a closely guarded secret. He is the owner of a mysterious stone passed down through generations, which grants the user and their family the ability to enter a dimension called Stasis, a world where time has completely stopped. His background is rooted in the responsibility of safeguarding this dangerous power, a duty he has shouldered for decades. It is revealed that he used the stone at least once before in the past to grant his granddaughter, Juri, a few more precious moments with her dying dog, an act that showed his deep care for his family but also demonstrated the stone's perilous potential.

In terms of personality, Grandpa Yukawa is portrayed as a gruff but deeply well-intentioned individual. While he can be cantankerous and is the "grampiest grampa," he is also a formidable figure who acts decisively when his family is in danger. He is pragmatic and wise, understanding the severe consequences that come with using the stone; this is why he kept its existence a secret from his family for so long. Despite his age, he does not hesitate to lead the charge into the perilous world of Stasis, showcasing a quiet bravery and a strong moral compass that prioritizes the safety of his loved ones above all else. His motivations are singular and clear: to protect his family. When his unemployed son Tsubasa and young grandson Makoto are kidnapped for ransom, he immediately takes action, using the stone to stop time and mount a rescue.

His role in the story is that of a catalyst and a guide. He is the one who introduces the family to the world of Stasis and its rules, including the dangerous monsters known as Heralds that enforce the dimension's natural laws. He fights alongside his granddaughter Juri and son Takafumi, acting as a key member of the rescue party. Unlike the more impulsive members of his family, his experience with the stone makes him more cautious and aware of the threats they face. His primary relationship is with Juri, his granddaughter. They share a powerful bond forged in the world of Stasis from her childhood, and she is the only one who initially shares his understanding of the stone's past. Their relationship is a partnership of mutual respect, with Juri often being the one to complete the mission after her grandfather secures their path.

While Grandpa Yukawa does not undergo a dramatic internal transformation, his role evolves as the story concludes. After the primary conflict with the cult leader Junji Sagawa is resolved, he accepts his part in ensuring the family's future. In a poignant moment, he voluntarily returns to the real world from Stasis, trusting Juri to handle the complex final stages of their plan. This act represents a passing of the torch, acknowledging that his granddaughter is now capable of carrying the family's burden.

Grandpa Yukawa possesses two notable abilities, both related to the mystical stone. The first, and most fundamental, is his capacity to activate the stone, which envelops anyone in close proximity in a black sphere and sends them into Stasis as beings known as Spectres. The second is a unique and powerful skill, as he can teleport himself and anyone he touches over short distances within the frozen time of the Stasis world. This ability makes him a highly effective and formidable fighter, often allowing him to gain the upper hand against enemies who can also move in the frozen world.