So, 'No.6' really hinges on two people for me. Shion is the boy genius who grew up sheltered in the elite Chronos district. His entire world gets turned inside out when he helps a fugitive named Rat—real name Nezumi—and gets exiled from the city. Their dynamic is the whole engine of the story. Nezumi is this fiercely independent, cynical guy from the slums outside the wall, and watching Shion slowly chip away at his hardened exterior is everything. They're opposites in every way, which makes their reliance on each other so compelling.
Honestly, beyond them, the other characters serve more to reflect aspects of the city or push the plot. Shion's mother is a window into the compliant citizenry, and Safu, his childhood friend, embodies the cost of blind faith in No.6's system. But the heart of it is always Shion's journey from naive idealist to someone who understands the rot at the city's core, and Nezumi's parallel struggle with his own hatred and vulnerability. Their relationship is messy, fraught, and deeply human against this sterile dystopian backdrop. I keep coming back to their final scenes together long after finishing the books.