I'm writing a short story about Nyaminyami and, like you have found nothing on the web that can throw any light on her name or whether she's a god/spirit like him.

Having researched it enough for 'artistic licence' I've found a Chitonga dictionary and given her a name myself, which is probably not too idiomatic but at least describes her a little - Meenda-Musimbi, which translates as water-girl lol