Nezuko remains alive in the Demon Slayer manga. The author chose to keep her alive as she is an important character with a vital part to play in the overall narrative.
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.
A great plot idea could be that Nezuko develops a new power that only works when she and Tanjiro are in close proximity. This power could be crucial in fighting a powerful upper - rank demon. For example, she might be able to create a protective shield around them both. Another could be a fanfiction where Tanjiro and Nezuko are invited to a demon slayer gathering in a far - off land. There, they meet other slayers from different regions and learn new techniques. Along the way, they also have to deal with some demons who are trying to infiltrate the gathering.
Well, Nezuko passes Sanemi's test through a combination of factors. Firstly, her strong willpower helps her stay calm and composed. Secondly, her bond with Tanjiro gives her the strength to overcome the challenges. Finally, her inherent kindness and desire to protect others shine through, convincing Sanemi.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.