Juuzou Hotoki

Juuzou Hotoki (仏木) is Maria's assistant and the one who helps her with her research. He's actually Maria's older brother. His real name is Makoto Imari.

Background
For most of his life, Makoto lived with his father overseas. When he was 11, he returned to the Imari household and met his sister Ai. He was repulsed by her lack of empathy and obsession with dissecting animals. Two years later, the family couldn't take Ai's strange hobbies anymore and sent her to live with her grandfather, Genjurou Imari.

However, the day before Ai was sent, she discovered Makoto's secret. Ai discovered her brother's porcelain dolls and realized he masturbated to the dolls. She also concluded it was due to an oedipus complex regarding their mother. Ashamed upon his secret being discovered, Makoto threw a teapot full of boiling water on her face, scarring her face forever.

Makoto apologized for the accident, but Ai didn't hold any resentment towards him. Rather than that, she had realized she wanted to mutate and become a monster, in order for her body to change. Some time later, while living with her grandfather, she found a pair of monsters in larvae form in the forest, the Pupa monsters. This picked her interest and for years she dissected them in order to know about them, with the help of her grandfather's associate, Chiharu Bizen.

Meanwhile, Makoto had developed incestuous feelings towards Ai, but she was too fixated on the Pupa monsters to notice his feelings. Ai had decided to use the Pupa monsters to fullfill her dream of becoming a monster. So she and Makoto decided to change their identities, becoming Maria and Hotoki.In order to continue being alongside her, he stays as his assistant.

30 years later, the male monster escaped from the laboratory and the female monster impregnated Sachiko Hasegawa and reincarnated as a human. Ai continued researching with what was left of them. It seems that she and Bizen had their differences when it came to their methods of researching, so Ai went on researching on her own with her followers.

Manga
For most of the story, Hotoki is always alongside Maria, staying as his faithful assistant and helping with her research. After Utsutsu and Yume escape Maria's laboratory, Maria gets a sample of their DNAs in order to create a Pupa larvae. She impregnates herself with it and proceeds to go to the island to assimilate Yuu, Utsutsu and Yume with her.

Yuu dies, but Utsutsu and Yume succesfully escape. However, they can't stop the corruption in their bodies anymore, so they proceed to eat each other and dissolve into butterflies in order to escape the corruption. Without the Pupas, Maria returns to normal. But now she's a monstruous blob connected to the island. However, she's happy with this outcome, since she got to became a monster like she always wanted. Hotoki finds the unconscious Maria and reveals his feelings towards her, expressing that the Pupa larvae that Utsutsu and Yume left behind can be their child. Maria coldly rejects him, insisting that she only loves her work and experiments. But she also admits it would have been not bad if she has a child with him someday. They finally call each other by their real names.

The "Maria's Island" afterstory shows everything about Hotoki's past with Maria, revealing that they are siblings and he hold incestuous feelings towards her. After the events of the series, Maria continues to live as a monster connected to the island and Hotoki has moved to the island in order to be with her forever.

Anime
Because the anime was short, it only covers the first part of the manga, and Hotoki is shown as Maria's assistant and voice of reason. His motivations and relationship with Maria are never revealed or explained.