K is the primary antagonist in the series, also known by his real name, Kevin Wayfair. He serves as a captain within the ranks of the criminal organization Equipment Smith, an adversary to the students of Kouryou Academy.

His background is rooted in tragedy. K is originally from America and had an older brother named Ryan, with whom he shared a very close bond. The trajectory of his life changed irrevocably when his brother died protecting him from an unknown assailant. This event led to the destruction of his family and pushed him into a dark and violent world. He was eventually sold to Equipment Smith, where he was raised as a soldier in an environment that valued only power and results. In this setting, he was shaped into a ruthless operative, with his brother's death serving as a deep-seated motivation for his power-hungry nature.

In terms of personality, K presents a deceptively calm and confident exterior, often appearing easy-going when confronting enemies on missions. However, beneath this composed surface lies a sadistic, vicious, and conniving individual. He is willing to do whatever it takes to achieve victory, which includes injuring rivals for an advantage or even killing his own subordinates when he sees fit. He enjoys playing psychological mind games with his opponents to break their will to fight. As a person driven by an intense craving for power, K holds a deep contempt for friendship, which he views as a sign of weakness, particularly focusing on a person's inability to protect others from danger. This philosophy puts him in direct opposition to the protagonist, Tooru Kokonoe, whose dedication to protecting his friends he openly mocks.

His role in the story is that of a recurring enemy sent to carry out Equipment Smith's missions. He is directly involved in attempts to capture specific individuals, notably targeting Lilith Bristol and Sakuya Tsukumo to fulfill his orders. His missions bring him into repeated and violent conflict with the main characters of Kouryou Academy.

K's key relationships are all defined by his role as an antagonist and operative. He is subordinate to Edward Walker, the true identity of Equipment Smith, who serves as his supervisor. It is Edward who provided K with a Unit to grant him the power he craves, and K seems determined to fulfill his orders at any cost, possibly out of a sense of gratitude. His primary rivalry is with Tooru Kokonoe. He is extremely disgusted by Tooru's resolve, which he feels resembles his own past self before his brother's death. He is willing to harm Tooru's friends both physically and mentally to break him. In his mission to retrieve Lilith Bristol, he speaks to her with respect and courtesy but does not hesitate to use cruel methods. When ordered to take Sakuya, he confronts Lilith again, knocking her unconscious to complete his task.

Throughout the series, K shows little development in a positive direction, remaining a dedicated and dangerous instrument of Equipment Smith. He is driven by his past trauma and his resultant obsession with strength, and he maintains his antagonistic stance against the academy students.

K possesses notable abilities as a fighter. He is always seen wearing a Unit costume, a powered suit that enhances his combat capabilities. This equipment provides him with significant physical power and access to special attacks, including powerful laser beams. In later confrontations, he receives an upgrade to his Unit, further increasing his threat level. He is a formidable opponent, relying on both his physical prowess and his psychological manipulation tactics.