There are some fond memories they share, and Kuukaku remembers all of them.
Like the time Soi heard of some of the odd customs the living practised.
So when Soi presented her with a box of chocolates and burnt fingers, Kuukaku had ticked her off and told her not to do such silly things; what was the point?
But she didn't have to explain why a box of chocolates showed up on Soi's desk a month later, the chocolates in shapes resembling bees and fireworks.
And when Soi hungrily licked the chocolate off her, no more words were needed.