Kaien Cross is the headmaster of Cross Academy and a central supporting figure throughout the story. He is the adoptive father of Yuki Cross and the guardian of Zero Kiryu, having taken both into his home. His background is marked by a dramatic transition; he was once a legendary vampire hunter known as the Vampire Without Fangs, considered one of the most powerful hunters in history. Due to an unusually high concentration of vampire DNA in his system, a consequence of having absorbed his twin in the womb, Kaien has aged extremely slowly. Although he is over two hundred years old, he maintains the appearance of a man in his mid-twenties. In his youth, he was a far more serious and grim figure, but a pivotal encounter with the pureblood vampire Juri Kuran led him to abandon his violent path and dedicate himself to her dream of peace.

Kaien's personality is a study in contrasts, blending a cheerful, eccentric, and often childish exterior with a deep well of seriousness and tactical wisdom. He frequently engages in playful antics, such as wearing oversized lion-shaped slippers or colorful hats, and enjoys teasing his adopted children, particularly Zero, whose grim reactions he finds amusing. He is prone to dramatic, tearful outbursts over minor frustrations, which serves as comic relief. However, this lighthearted facade masks a fiercely protective and calculating individual. When a genuine crisis emerges, Kaien sheds his eccentric persona completely, becoming a stoic, decisive, and formidable protector. He is a master of omission, often keeping crucial secrets from Yuki and Zero to shield them from pain or danger, a trait that stems from his complex moral compass.

His primary motivation is the establishment of lasting peace and understanding between humans and vampires, an ideal he inherited from Juri Kuran. To this end, he founded the Night Class at Cross Academy, allowing vampires to study alongside humans during the night. He acts as a bridge between the two worlds, maintaining a difficult balance between his duties as headmaster, his role as a former hunter, and his relationships with powerful vampires. In the story, he serves a largely observant and supportive role initially, but becomes a more active player as the conflict escalates. His key relationships are deeply personal. As Yuki's adoptive father, he is doting, proud, and unconditionally loving, having known the truth about her pureblood heritage from the beginning. His relationship with Zero is more complex; he acts as a mentor and surrogate father, refusing to give up on him even as Zero descends into vampirism. He also shares a unique bond with the pureblood Kaname Kuran, whom he trusts to a certain extent, knowing Kaname's parents and feeling indebted to them. He is also an old friend of the pureblood Isaya Shouto.

Kaien's character undergoes significant development from a retired legend to an active leader once again. He initially upholds a strict personal vow of non-violence, refusing to kill vampires to protect his dream of peace. However, when the Cross Academy is directly attacked, he sets aside this vow to defend the Night Class, reclaiming his role as a hunter and even killing the acting president of the Hunter Association to protect his students. Following this event, he is briefly held accountable by the Hunters but eventually returns to lead both the school and the Hunter Association, actively recruiting Zero as his successor and preparing for a larger, more dangerous political landscape. As for notable abilities, Kaien is an immensely powerful fighter, whose skills earned him legendary status among hunters. He possesses superhuman strength, speed, and reflexes that surpass most vampires. He also has an acute sense of smell and can wield anti-vampire weapons, his personal weapon being a sword concealed within a cane. His unique vampire DNA grants him an aura that can be mistaken for a vampire's and renders him practically ageless.