Gender: Male Personality and Traits: Unlike Yodomi or Professor Rokujo, he is a kind person who genuinely wants to bring happiness to people. However, he is mentally fragile and highly emotional, often bursting into tears if his books are treated poorly. Relationship with Zenitendo: He is not hostile toward Beniko, the owner of Zenitendo; rather, he feels grateful to her or is in a position of receiving her help. He once told Shoko, who bought "Delicious Soda" from Zenitendo, that the drink wouldn't make her happy and offered her a choice from his own books. Shoko chose "The Recipe for Calling Happiness" and found joy. After Shoko left, Zenji pushed the "Delicious Soda" into a corner, thinking he should return it to the person who made it (Beniko). He also told Reina, who bought "Friend Blend" from Zenitendo, that it wouldn't make her happy and recommended "Best Friend Exchange Diary," which led Reina to find a true friend. To Naoto, who wanted his wife to like reptiles, he gifted the "Lovely Reptile Encyclopedia," but it backfired; his wife, Yuiri, became so obsessed with reptiles that she nearly committed a crime. An enraged Naoto tore the encyclopedia to shreds and threw it in the trash, but the book flew through the sky the next day and returned to its original owner, Zenji. Shocked by the tattered state of his book, Zenji closed his shop and hid under his covers. He runs the Senboku Bookstore.

Gender: Male Zenji's twin brother and the owner of Senboku Bookstore. While he looks identical to Zenji, his personality is the complete opposite; he has sharp eyes and is prickly toward anyone who isn't his brother. Unlike Zenji, who holds no grudge against Beniko, Kaichi does not think highly of her for breaking Zenji's spirit. He is a sorcerer who deals in curses. He visited Zenji to congratulate him on opening the new shop, only to find it closed because Zenji was traumatized by the customer who trashed the "Lovely Reptile Encyclopedia." Hearing this, Kaichi told him, "Don't cry with a face that looks just like mine. I'll get revenge for you." Zenji stopped him before he could, though. He declared he would help with the shop, questioning how Zenji could ever run a business when he collapses from shock every single time.