Kaien Cross is the headmaster of Cross Academy and holds the position as the head of the Vampire Hunters Association. He is the adoptive father of Yuki Cross and the guardian of Zero Kiryu. Despite being over two hundred years old, he appears to be a man in his mid-thirties. He has long, straw-colored hair that he usually pulls back into a ponytail, and he is often seen wearing eccentric clothing, such as oversized lion-feet slippers, large colorful hats, and a pink apron. He also wears spectacles, though he removes them in moments of serious action.

Before his time as an educator, Kaien was a legendary vampire hunter known as the Vampire without Fangs, a title earned due to his immense strength and skill. He was considered the best hunter of his era. His physiology is unusual, as he possesses an abnormally high amount of vampire genetic material, which has halted his aging and gives him an aura that can be mistaken for that of a vampire. In his prime, he had a more serious and mature demeanor. A pivotal event in his past was an encounter with the pureblood vampire Juri Kuran. After he foolishly agreed to hunt her while she was vulnerable, she spared his life and forgave him. This act of mercy led to an unexpected friendship, and Kaien ultimately retired from hunting to take on Juri's dream of building a school where humans and vampires could coexist. Sixteen years before the story begins, he established Cross Academy with that very goal in mind.

Following the deaths of Juri and her husband Haruka, the pureblood Kaname Kuran brought their hidden daughter, Yuki, to Kaien for safekeeping. Kaien adopted Yuki, becoming her doting, if slightly odd, father. Several years later, he also took in Zero Kiryu after the murder of Zero's parents by a pureblood vampire. He has always told only half-truths to protect Yuki and Zero, keeping his knowledge of Yuki's true past a secret from her.

Kaien's personality is outwardly cheerful and eccentric. He is prone to tearful dramatics over his perceived failings as a parent and enjoys teasing his children, particularly the reluctant Zero, often joking about his vampire nature. However, this lighthearted facade masks a deeply serious and protective individual. He is proud of and fiercely protective of both Yuki and Zero. While he dreams of peace, he is not naive, and he has proven willing to take direct and violent action when necessary, despite having sworn off harming vampires.

As headmaster, Kaien's primary role is to oversee the Night Class, an experimental program created with Kaname's assistance to promote understanding between the human Day Class and the vampire Night Class. He appoints Yuki and Zero as the school's Guardians, responsible for maintaining the peace between the two groups. Throughout the story, he acts as a mentor and observer, but he becomes more directly involved when the Hunter's Association attacks the Night Class. During this conflict, he defends the vampire students and ultimately kills the corrupt president of the association.

Kaien shares a complex bond with Kaname Kuran. He is deeply indebted to Kaname, both for Yuki's safety and for his assistance with the Night Class. He is also friends with another pureblood vampire, Isaya Shouto, whom he used to intentionally pick fights with in the past.

Kaien possesses notable abilities as a result of his heritage and training. He is a master vampire hunter, wielding a sword hidden within a cane as his primary anti-vampire weapon. He has enhanced strength, speed, reflexes, and an acute sense of smell for detecting vampires. His most distinctive trait is his extremely slow aging, having remained youthful for over two centuries, and the unique vampire-like aura he emits. His development in the story sees him transition from a cheerful, passive observer to an active participant. Despite his earlier distaste for hunting, he takes up the mantle of leader of the Hunter's Association again, all while continuing to work toward his founding ideal of coexistence and protecting the two children he raised as his own.