Kei is the protagonist of the story, a high school boy who leads a double life balancing his academic studies with a secret career as an underground mahjong player. He is known in the underworld by the moniker K of the ICE, a nickname that reflects his cold-hearted, calculated strategies and stylish, ruthless gameplay at high-stakes parlors.

His background is marked by a profound childhood trauma. In elementary school, his younger brother, Takashi, was murdered by their parents, who staged it as a drowning accident to collect insurance money. This devastating event fundamentally shaped his personality and world view. Several years later, during the summer of his first year of high school, an encounter with a man named Macchan introduced Kei to the dangerous world of underground mahjong.

Personality-wise, Kei is defined by his exceptional calmness and composure. He is a realist who rarely shows emotion, approaching situations with a steady, logical mind. His defining traits are a frigid, analytical demeanor at the mahjong table and an unwavering focus, which contribute to his fearsome reputation. His internal drive is largely centered on a desperate need to protect the people he cares about, a motivation that can become a vulnerability his enemies exploit.

Kei possesses an extraordinary memory and a highly theoretical, analytical playstyle. He is a brilliant strategist who relies on calculation rather than luck, a skill that allows him to dominate opponents despite his age. His signature catchphrase is "I'm trembling," which hints at the intense pressure he feels despite his outward stillness.

Throughout the story, Kei’s most significant relationship is with Amina, a stateless girl from a foreign country whom he rescues and takes into his home. His bond with her is unique, as his usual composure crumbles when her safety is threatened, making him act with desperation. Another key relationship is with Yuu Katsuragi, his childhood friend. She is deeply involved in his life and is ultimately revealed to be his future spouse.

Kei’s role in the story is that of the central player navigating the violent and unforgiving criminal syndicates that control underground mahjong. He begins as a substitute player for the Sakurawa Association, a path that leads to brutal physical consequences, including the loss of the little toe on his right foot as a penalty. His journey is one of constant escalation. In the Human Pillar Arc, he teams up with a rival named Noriyuki Takatsu to face the antagonist Hatori. After losing a high-stakes game, Kei is forced to hang himself as punishment. He survives this ordeal but suffers permanent physical damage, often requiring a wheelchair or cane to move afterward.

His development is a continuous struggle for freedom and survival. After a period away from mahjong, he is drawn back into the underworld to obtain the Saturday Meeting Records, a tool he believes will secure his liberty. By winning the 28th Ryuuou Title Match, he acquires this prize as well as the Hatori's List. To protect Amina and Yuu from the fallout, Kei accepts a position as the chief advisor of the Sakurawa Association, essentially trading one form of entrapment for another. In the final resolution, he marries his childhood friend Yuu and adopts Amina as his sibling, creating a new family unit as a refuge from the chaos of his life.