Valentina Venables is one of the six daughters of Rupert Venables' brother Will. Will is a fellow Magid, and older than Rupert. He and his family live on a lively, untidy farm on the world called Thule, which is Ayewards of Earth, so it's a magical place.
Valentina is described as the 'little lionhead' -- she was the one who kept shrilling to be told what use Maree Mallory's nails were, and Maree of course showed her by using them to nip the top off her soft-boiled egg. And that is about all we know about Valentina.