Ishabal Ladyhammer, nicknamed Isha in the imperial court, is the head mage of Berenene dor Ocmore, the Empress of Namorn. She is also considered a great mage and the Empress' most trusted advisor. Berenene listens to her advice and even asks her to convince Trisana Chandler to stay in Namorn, employed at the imperial palace, instead of returning to Emelan. For this Berenene left it to Isha's consideration which means she used to convince Tris.
Isha is very old and thus doesn't accompany the Empress on her regular rides and hunts. Instead her younger colleague Quenaill Shieldsman provides the magical protection for Berenene then.
The mage only appears in The Will of the Empress and is a medium ranked character in the book, due to the fact that she influences the plot in some points majorly, but not at all in other points, and because she is not mentioned much.