Zeno is Stella's beloved big brother and the oldest sibling of the Kuonji family.
At first, Zeno is shown to be a kind-hearted, loving older brother to Stella. After being murdered by Syd, and then ressurrected by his mother, his personality changed, and he became a cold-blooded murderer and seemed to not remember Stella anymore. With time, his memories came back, and he went back to be Stella's loving older brother, but he was still chosen to be the Executer to the Kuonji family, therefore continued to be a murderer.
His true identity is of the mysterious white-haired boy that used to bully Stella when she was a child in the orphanage. He was sent by the Washimiya Cult to the airplane with the mission of making the plane fall and kill the ones inside it, Zeno and Stella. Somehow, both Zeno and Stella, and also the white-haired boy, survived the accident. Stella and Zeno were brought to the Kuonji family's Mad Tea Party but contrary to Stella, who was able to drink the tea, Zeno seemed to be weakened by it, and showed signs that he was soon going to die. Stella begged for her future mother to revive him, and she promised to do so, but before she could heal him, the white-haired boy stabbed him, absorbing his memories, and said he was ready to assume his place, to become Zeno. Mother knew he was a member of the Washimiya Cult, and to prove his loyalty to the Kuonji family, he stabbed his own hand where the Washimiya Cult tattoo was placed, getting on his right hand a scar similar to the one the original Zeno had in his left hand. This way he became Zeno, with Stella never knowing about his true identity.