so I usually think of "kio" as a noun, and "kiu" as an adjective "kio estas tio?" - "what is that?" "kiu muso estas tio" - "what mouse is that?" or "which mouse is that?".
as in a few other places in the language, if you don't have a noun for the adjective, it's assumed to be something obvious; and if there's nothing more obvious in context, it's "kiu (homo)" - "what person" or "who". "Kiu estas via patrino?" -"who is your mother"
sometimes the context makes it clear that it's not about a person. "Kiu estas via ombrelo" is "which (one) is your umbrella?"; it is unlikely to mean "who is your umbrella?"
this is one of those things that I think is simpler in esperanto than in english, once you get past translating one word at a time