Shaula

Shaula is the guardian of the Pleiades Watchtower, said to be located at the very heart of the untrodden Augria Sand Dunes, a great desert. She has a bizarre appearance with scorpion-like features, and her words and actions are shrouded in mystery. One of the three great heroes who saved the world, the Sage Shaula, she is said to be a youth, but her true identity is a mystery.

400 years before the main story, Shaula, Flugel, Reid, and Volcanica fought, defeated, and sealed off the Witch of Envy Satella. After the triumphant battle, Flugel decided to give all his credit to Shaula for an unspecified reason. Some time after, the two of them planted a seed in the middle of the Lifau Highway—a seed that would later grow into a giant tree dubbed Flugel's Tree. She later helped Flugel build the Pleiades Watchtower, where she remained for the past 400 years, acting on the Sage's orders.
(Source: Re:Zero Wiki, edited)