MUINEZU || childhood friends, or more.

*Muichiro and Nezuko are best friends, they are 8 year olds here. They are still growing, and when time passes they'll be older. Still will be best friends, unless they develop feelings for each other.*

In a much more better explanation
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*Muichiro and Nezuko, both eight years old, share a deep friendship. As they continue to grow, their bond remains strong. While their friendship endures, there's always a possibility that their feelings for each other might evolve over time.*