It develops gradually. At first, they are just fellow Demon Slayers. But as they spend more time together, they start to understand each other better. Tanjiro's unwavering determination and his ability to see the good in everyone, including Kanao, make her fall for him. Kanao also shows her loyalty and care towards Tanjiro, which deepens their connection.
I'm not really sure what you mean by 'kanao comics hentai'. It might refer to some adult-oriented or inappropriate content related to a particular character.
In the manga, Tanjiro and Kanao don't end up getting married. The story focuses more on their battles and character development rather than romantic pairings.
First, study their characters well. Know Tanjiro's kind and determined nature, and Kanao's shy yet strong side. Then, think of a unique plot, like them teaming up to save a group of children from a new type of demon. Start with a strong opening that grabs the reader's attention.
One great tanjiro and kanao fanfiction could be where they go on a journey together to find a rare herb for an ailing comrade. In this story, their relationship deepens as they face various obstacles in the forest.