Iso

Iso is one of the Watchers of the Void, the Apostle of Animosity the Contractor of Kazabubu and the protagonist of the first Monument Book. Iso continues to be a referenced character though doesn't appear again until Monument 6 due to the nature of the Monument series containing serial yet self-contained stories. Iso has not been mentioned since Monument 10 but references to her being called the "Watcher that slew Greed" do appear in Monument 8 and 9. 
Iso led the School Group in Monument 1 through the Void and reluctantly taught them how to combat the Voidal threat, believing that it was ignorant to leave them without a means to defend themselves and an increasing realization that she needs allies. She continued to fight against the Agents of Greed and in Act IV finally confronted and killed The Monument of Greed.