As an immature thymocyte he went through training at Thymus school, where apprentice cells like himself learn to become Mature Thymocytes. Although in the beginning he was scrawny and weak, he never gave up.Killer T Cell is loud, obnoxious, and quick-tempered, resulting in rivalries with many cells. He also despises it when a non-white blood cell forms a friendship or relationship with one of his kind.