Originally an orphan boy from the slums, he was kidnapped as Subject 17 of the illegal chimera bio-lab T.D.A. In his early teens, he was injected with DNA from various Demon Realm plants and creatures, living his entire life as their slave. After decades of overwork, as his lifespan reached its limit, he was ordered to buy time inside a Gate and die so the other Hunters could survive. However, he successfully resurrected due to the power of an immortal Demon Realm creature within his body. After confirming that the test subject mark and self-destruct spells had vanished, he made the Demon Realm his home, built a house, and sought to live a healing life. His main weapons are transforming his arms into blades or using the power of an S-rank Demon Realm creature called the Silk Thread Caterpillar. Having lived as a Chimera Hunter and received treatment as the top subject of mad researchers, he also serves as an intellectual who knows everything in the story. Despite his past, he is helpful to others.

The first Demon Realm creature Cheon Do-un encountered after his resurrection. When they met, it had already died once due to poison, but being an immortal-type monster, it barely survived by transforming into a long-tailed tit form, which is most suitable for survival. Since it cannot speak and only coos like a pigeon, Kim Na-ri named it Guu. It is also a psychic ability user and can communicate with Cheon Do-un, who shares similar powers, through telepathy.

A mechanical organism from a chimera lab called Hanbit Research Institute. She has the ability to mimic any form. Although she is a mechanical chimera, she clearly possesses emotions.