Kaien Cross serves as the headmaster of Cross Academy and is a central supporting figure in the story. He is a tall man with blond hair that he typically pulls back into a ponytail, and he wears glasses. His appearance is often unconventional, as he has a habit of wearing eccentric clothing, such as oversized lion-feet slippers and colorful hats, which contributes to his seemingly unserious demeanor. Despite being over two hundred years old, he appears to be a man in his mid-twenties, a result of unique circumstances in his past.

His personality is marked by a sharp contrast between his public behavior and his true nature. On the surface, Kaien is cheerful, eccentric, and prone to dramatic, tearful outbursts, especially regarding his role as a father figure to his adopted children, Yuki Cross and Zero Kiryu. He enjoys teasing Zero, often pretending to be heartbroken by the young man's refusal to see him as family. He is also known for cooking bizarre meals and insisting that Yuki and Zero try them. However, this jovial exterior hides a deeply serious and protective individual. He is known to lie by omission, keeping crucial secrets about Yuki and Zero's pasts in what he believes is their best interest, demonstrating a paternal instinct that prioritizes their safety over their comfort.

Kaien's motivations are rooted in a profound personal transformation. In his youth, he was a legendary vampire hunter, known as the Vampire without Fangs, and was considered one of the most powerful in history. He was born with an unusual amount of vampire genetic material, which halted his aging and granted him extended longevity. His perspective changed after an encounter with the pureblood vampire Juri Kuran, who spared his life and forgave his hostility. Inspired by her dream of coexistence between humans and vampires, he retired from hunting. He founded Cross Academy and established its unique Night Class for vampires, aiming to prove that peace between the two species is possible.

In the story, his primary role is as the visionary and administrator of Cross Academy. He maintains the fragile peace between the Day Class (human students unaware of the truth) and the Night Class (vampire students). While he is always aware of the events unfolding at the school, he largely entrusts the discipline and protection of the student body to Yuki and Zero, who serve as the school's Guardians. His position places him as a bridge between the human world of Vampire Hunters and the vampire society.

His key relationships define his actions and development. As the adoptive father of Yuki, he took her in after her parents died and she was brought to him as a child by Kaname Kuran. He knowingly raised Yuki, concealing her true identity as a pureblood vampire to protect her from her dangerous past. Similarly, he took Zero in after the murder of Zero's family by a pureblood, attempting to slow Zero's transformation into a vampire. He harbors a deep sense of responsibility for both of them, viewing them as his cherished children despite their complex natures. He also shares a unique bond with Kaname Kuran, having known Kaname's parents. He is indebted to the Kuran family and collaborates with Kaname to maintain the Night Class, respecting Kaname's power and influence while also expressing concern when Kaname's actions become too ruthless. Furthermore, he maintains a friendship with the pureblood Isaya Shouto, having once deliberately picked fights with him.

Throughout the series, Kaien undergoes significant development. Initially portrayed as a comic relief figure, his past as a formidable hunter is gradually revealed, adding layers of gravity to his character. When the school is attacked by hostile vampires, he abandons his pacifist stance to defend the Night Class, even fighting and killing the corrupt acting president of the Hunter Association. Following this event, he is taken into custody but eventually returns to serve as both the head of Cross Academy and the acting president of the Hunter Association. He transitions from a passive observer to an active player, vowing to stand between Yuki and Zero should they ever become enemies.

Kaien possesses notable abilities rooted in his past as a legendary hunter. He is an exceptionally skilled combatant, powerful enough to defeat high-ranking vampires and even the head of the Hunter Association. His primary weapon is a sword, the exact nature of which is linked to his hunter heritage. He also possesses knowledge of ancient vampire ceremonies; for example, he performs a ritual that allows Yuki to metaphorically tame Zero, creating a psychic bond to prevent Zero from degenerating into a mindless Level E vampire. His most unusual ability is his agelessness and resilience, as he has survived for over two centuries without visible aging.