No, the story of Kannagi is likely fictional. It's usually created for entertainment or to convey certain themes and ideas rather than being based on real events.
The decline in popularity of Kannagi manga might be because the characters weren't developed enough to keep readers emotionally invested. Another factor could be poor marketing and promotion, resulting in less visibility in the market.
One great 'kazuma and darkness fanfiction' is 'Adventures Beyond the Known'. It delves deep into their relationship as they face new challenges in a land far from their usual quests. The story beautifully portrays Darkness's unwavering loyalty and Kazuma's growth as a character.
Tappei Nagatsuki is notable for writing 'Re:Zero − Starting Life in Another World'. His writing style is able to convey complex emotions and the development of the story's mystery is really captivating.
Another might be 'Re:Zero − Starting Life in Another World'. It stands out for its unique premise of the protagonist having to relive his experiences in a different world over and over again, leading to a complex and deeply emotional story. The character development is really top - notch.
A common theme is the clash of different worlds. Since it's a crossover, kazuma mikura often has to deal with the differences between his world and the world he crosses into. This can create a lot of interesting storylines.