The Seventh Sister is a Mirialan Inquisitor introduced in Star Wars Rebels' second season, in "Always Two There Are" (2015), voiced by Sarah Michelle Gellar. She operates in partnership with the Fifth Brother.
She is written as the calculating one — she uses droid probes, gathers intelligence, and takes an obvious pleasure in extending an encounter she could end. Her interest in the child Force-sensitives the Empire is collecting is the thread that ties the Inquisitorius to the wider purpose of the programme. She died on Malachor in 3 BBY. Canon.
Catalogue entry. This figure has a verified record here rather than a full write-up — the encyclopedia labels its own depth instead of padding it.