Souhei Shima (島草平 Shima Souhei) is the main character of the Invisible novel. He had his first debut in Invisible, although he was not shown in the official PV.
- Souhei has light brown and slightly messy hair and brown eyes. He is described to have a "puppy-dog" face. Souhei wears school uniform that consists of a white blazer that has sleeves, shoulders, pockets, and front folds lined with gold, a grey dress shirt, black or brown tinted dress pants, a black or brown tie, brown shoes, a black belt, and a star tiepin. For the entire novel, he is not seen with other attire.
- When he was younger, Souhei lost contact with his parents and went to go live with his Aunt- a woman that is not fit for taking care of children and dislikes Souhei, forcing him to pay rent to live in her apartment. He has a cat called Othello.
- After having an argument with his former best friend Hijiri Aota, Souhei found himself isolated in his class. He has no friends and he spends his lunch times reading books outside on the courtyard.
- Being isolated from the class caused Souhei to hold the strong desire to become invisible, which is granted by Maki-chan. He later travels to the transparent city, where he meets Seiji and Riina. Seiji offers him a home there with him and Riina.