So in my au these two are besties due to a few reasons, but the main reason is that they basically did everything together.
They both went to the same final section together they already knew each other as kamado tanjiro was both of their teachers (senjuro originally learned from his father but once he became an alcohol addict he started training with the new sun hashira and his sister.)
Nezuko and senjuro became hashira together as they fought lower moon 3. During that fight Nezuko had recently become a demon (about 2-3 months pre lower moon fight) and had use her body to protect senjuro from death on instinct. (That night was also the night she became sun immune).
There breathing styles work well together as they boost the each other. the only other hashira that is better with nezuko in terms of team work and collaboration between breathing styles is genya and tanjiro.
Similar age, even tho senjuro was born somewhere in January-February the year after nezuko was born the age gap is small and they can still bond pretty easily as they were both in the same generation.
Her, senjuro and muichiro are in a trio of the 20-year-olds lol