Hitotsume-sama is a Zingai that is seen in many of Eve's music videos. He is also referenced in Kara no Kioku multiple times despite only appearing very briefly in the first few chapters.

He is 200 cm (6'6") tall, however it is unknown if his hat adds to his height. He has no facial features other than the large single eye on his face. His head is cylinder-like, and his arms are thin. His hands are large, and with long fingernails. He is usually seen with four fingers but has the ability to change the amount at will. He is also able to do this with his toes.

Clothing-wise, Hitotsume wears all black and white. His outfit consists of a top hat, a black form-fitting robe, a white shawl, black leggings, and black boots. In Dramaturgy (One of Eve's music videos), he is seen wearing a tailcoat instead of a robe, and his top hat has a small bunch of feathers on the side. Occasionally, he wears a black cloak on top of his attire.

On the day of his high school entrance ceremony, Rei is suddenly assailed by a feeling of languor. When he wakes up, he finds that his surroundings are filled with things that could not be called human. In a town where strange phenomena continues to stretch, a group of lonely youths have their encounter. 

Music video for Eve's "Literary Nonsense". 

The music video for Eve's single Kokoro Yohou, created as a commercial for Lotte's Ghana brand chocolates and theme song to Lotte's 2020 Valentine's day campaign "PINK VALENTINE". The upbeat video follows three friends as they prepare to confess to their respective crushes on Valentine's Day—using Ghana Chocolate, a line manufactured by Lotte in Japan. 
(Source: Crunchyroll, edited)

Music video animated by Kukka for the song Gunjou Sanka by Eve featuring Hatsune Miku. This song was made to celebrate the first anniversary of the Project SEKAI COLORFUL STAGE! feat. Hatsune Miku rhythm game. 

Note: There exists two versions of this song; one by Eve ft. Hatsune Miku on Eve's YouTube channel, and the other a Hatsune Miku only version on Eve's Niconico account, which was released on the same day.

Tobi Otogiri, a second-year student in middle school, doesn't spend much time with his peers. Instead, he mostly chats in secret with his pal Baku, a mysterious backpack he received the day he last saw his older brother. Or at least, Tobi thought his chats were secret until a girl in his class named Ryuuko suddenly mentions Baku. Soon, strange things start happening at school, and the two of them realize the cause might be a creature only they can see… Thus begins an adventure blending reality and fantasy, human and nonhuman—a story about life eating life.
 
(Source: Yen Press)

Music video animated by Mah for the song Dramaturgy by Eve.

Otogiri Tobi, a 2nd grade middle school student, spends his depressing days searching for his missing older brother alongside his inhuman companion, Baku. Tobi used to rarely speak to anyone besides Baku, however one day after his classmate Shiratama Ryuuko suddenly approaches him, his life begins to change...? This is an adventure surrounding the "human" and the "inhuman" that a lone boy faces. 
(Source: Comic Walker, translated)