From the story I'm writing called 'Tane’. A comic of the first time Yami [purple hair] completely possesses Hidarri's [yellow hair] body in the presence of their old mutual friend Rinku [black hair guy - belongs to an old friend]. Simply put, Yami is a demon that originated in an early strain of Hidarri's family genetics from decades ago. Yami [originally Eami Rîmaru] was always able to manipulate his host body as he was passed down the genetic line, but he was only strong enough to alter the DNA of his host body with the help of Hidarri's untapped power. It's not a comfortable transition, as you can tell. Hidarri wasn't strong willed enough to keep Yami pushed back in his subconscious, and Yami occasionally took possession of Hidarri while he felt emotionally compromised or while he was dealing with difficult emotional turmoil. In this case, it was Yami's romantic feeling for Rinku that Hidarri confused for his own, which caused him deep uncertainty toward his sexuality, and weakening his will enough for Yami to finally push his way to completely take over his host's body. **Correct me if my Japanese is wrong!!