Zen is a poisonous bird yōkai of the Yakushi Clan, affiliated with the Nura Clan. He stands 177 cm tall and weighs 64 kg, with khaki hair fading to black at the neck, pale skin, and deep red eyes featuring small pupils. He typically wears a purple kimono paired with a dark blue haori adorned with red details on the sleeves and collar. Born with a naturally weak constitution due to his species' short lifespan, Zen grows progressively weaker with age. He specializes in medicine, wielding extensive knowledge of herbs and plants to create diverse salves and healing compounds. His abilities center on manipulating poisonous feathers from retractable wings. These feathers inflict adverse effects on contact or can be projected like arrows. During "matoi" rituals, he uses them collaboratively to enhance Rikuo Nura's attacks with a poisonous aura. While primarily offensive, his feathers can occasionally facilitate healing in certain adaptations. Zen and Rikuo Nura are childhood friends and sworn brothers, having cemented their bond by exchanging a 50:50 sakazuki cup. This relationship began after "Night" Rikuo rescued Zen from a betrayal by his subordinate Hebidayu, who attempted to usurp leadership of the Yakushi Clan, forging Zen's unconditional loyalty. Despite his poor health, Zen frequently insists on participating in battles, leading Rikuo to deliberately exclude him from mission plans for protection—a recurring dynamic that often ignites Zen's temper. His personality is hot-headed and excitable, causing him to vomit blood when intensely emotional or angered. Yet, he demonstrates calmness when necessary, particularly in medical or strategic situations. Across key conflicts, Zen's role shifts from medical support to active combatant. During the Shikoku arc, he tends to Rikuo after a collapse and later joins battles in adaptations, using his feathers for healing. In the Kyoto arc, he stows away to aid Rikuo, intervenes in disputes among allies, and provides medical assistance during training. He defies Rikuo's orders to evacuate during an attack, declaring his desire to fight alongside the Hyakki Yakō. This resolve culminates in their collaborative matoi ritual. Zen later participates in the assault on Nijō Castle. By the Shibuya arc, his declining health confines him to healing injured clan members at headquarters. Following the series' main events, Zen acknowledges to Shoei that battle strains have accelerated his deterioration, and he anticipates an early death.