One idea could be a rengoku fanfic where he trains a new group of young slayers. It could focus on his unique teaching methods and how he instills his values in them.
Tanjiro would be a great character to pair with rengoku. Their different personalities could create an interesting interaction. Tanjiro's kind - hearted nature and rengoku's fiery determination could lead to some great storylines.
It might also be a term of endearment for a younger male friend or acquaintance. In some groups, people use 'brother' in a friendly way, and 'little brother' could be a specific person within that group who is no longer interested in stories.
There's not enough information to say exactly who the little brother is. He could be an ordinary boy in a family who is the youngest among his siblings.
Here are some novel recommendations about the brother's cute younger brother:
1. " The Charm King's Addicted Pet: The Crazy Concubine."
2. " Separated by a Wall: Addicted to Addiction by a Male God."
3. " Gu Fei Meng Ye: Addicted to a Superstar "
4. [Little Sweetie: Enchanting Summoner]
5. " The Addicted Demon Wife: The Fox King's Wife Is Really Gorgeous "
6. [Pet Addiction: Young Master Di's Strong Love]
7. "The Twin of the Storm."
8. [God Descends From Heaven: My Four Older Brothers Spoil Me]
These novels covered the plots of the older brother's love and affection for the younger brother. There were different types of stories such as modern romance, aristocratic families, romantic youth, and so on. I hope you can find something you like in these recommendations. Please note that the novels recommended above are based on the search results provided and may not be comprehensive or accurate. It is recommended that you check the book's details and reviews before reading to ensure that it meets your preferences and expectations.