Their relationship can be further explored by focusing on their non - verbal communication. Since Nezuko can't speak much as a demon, the looks they exchange, the way Tanjiro gently touches her head to reassure her, these small gestures can be delved into. For example, in a dangerous situation, Tanjiro's glance at Nezuko could convey a whole world of trust and protection.
Tanjiro's character development would take a dark turn. He would start from being a pure - hearted demon slayer to a conflicted being. His internal struggle between his human memories and his new demonic desires would be the main focus. For instance, he might remember his promise to his family but be unable to resist the urge to harm others.