Zen Toono is a supporting character in the Mieruko-chan series. He is a substitute teacher for Miko and Hana's class after their homeroom teacher took maternity leave.
Zen has an averaged size build with short cropped, tousled hair that has bangs hanging on the left side, tired orange eyes and he is usually seen with an impassive expression. Many of his female students admire his looks, often asking to be walked home or have extra time spent with him.
He wears a button-up dress shirt with navy blue slacks and brown dress shoes.
Zen was initially portrayed as an emotionless, borderline sadistic sociopath who is heavily implied to be the culprit behind the recent string of animal abuse and killing. This was, at first, supported by the presence of a multitude of distorted cat spirits around him as well as a colossal spirit.
However this is later shown to be a misunderstanding. While Zen is stoic and blunt, he is a well-meaning individual who wishes to let go of his traumatic childhood but doesn't know how. He displayed enough selflessness to throw himself into the path of an incoming car to protect a kitten, despite the risks to him. In the same vein, he tracked down the actual cat murderer and successfully attacked him with a taser, as well as possibly killing him and hiding the body.
During his childhood, he was a meek and submissive individual who was kept under his mother's thumb.
In Chapter 21 of the Mieruko-chan manga, Zen is a young boy under constant pressure and verbal abuse from his mother, Mrs. Toono. When he was two points off from a perfect score on a test, she berated him to no end, rejecting his apologies as he wallowed in shame. At the same time, he was invited to a birthday party for his friend Ryota, but Mrs. Toono guilt tripped him out of it, calling him a dishonest child. She accused him of betrayal, and by going to the birthday party he was disappointing her, just like her husband did. Zen is convinced out of going, and he then goes to the park to destroy the hard-earned gift he bought.
At the park, he meets a stray cat that he takes home and cares for in secret. However, his mother discovers it and promptly kills it, discards its body and accusing him once again of being an ungrateful burden. Crying silently, he does nothing but nod. After his mother dies, he takes the job of taking care of strays in order to atone for the first one. Many are too weak to survive, with it being implied that his mother's spirit drained their life aura, but he views it as his duty since he couldn’t let go of his guilt. He cares deeply for the neighborhood animals, even going as far as tracking down the actual cat murderer and attacking him.
- The name Zen means "good/goodness, right, virtue" (善).
- Zen's surname Toono means "distant, far" (遠) (too) and "field, wilderness" (野) (no).