Many fanfictions focus on the character development of Connor and Markus through their relationship. They might start off with a lot of mistrust. For example, Connor might be suspicious of Markus' motives for starting the revolution. But over time, they could learn to respect each other. This could be shown through shared experiences, like facing a common enemy or making difficult decisions together. The fanfiction can take their relationship in many different directions, depending on the writer's interpretation of the game's world and characters.
You could also try Wattpad. While it has a wide variety of content, there are some really good Detroit: Become Human fanfictions there as well. Look for ones with high ratings and lots of comments, as those are usually signs of well - written and engaging stories. Additionally, Reddit has some Detroit: Become Human communities where people might share or discuss fanfictions about Markus and North.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a large collection of Detroit: Become Human fanfictions, including those about Markus. Just use the search function and type in 'Markus Detroit: Become Human' to find relevant stories.