Glariza was a member of the Demon Clan and in some point she became the lover of the great wizard, Gowther. She was part of a group of innocent Demons that were captured by the Goddess Clan

Goddess Clan

The Goddess Clan is an extremely powerful race able to stand on equal ground with the Demon Clan, although it required the cooperation of the fairies, giants and humans to seal them away after losing their strongest fighter. Despite their power, they needed physical forms to actually affect the world by their actions.

› wiki › Goddess_Clan

before being massacred by the Archangel, Mael.

Does Gowther have a love interest?

Slader appears to hold romantic feelings for Gowther, as he privately tells Simon he is incredibly attracted to the Goat's Sin. However, he was conflicted about his attraction to Gowther because of his status as a criminal who betrayed the king.

Are Gowther and Guila dating?

She became lovers with Gowther after the defeat of Hendrickson, but it was revealed to be due to Gowther's brainwashing. When Gowther released her from his brainwashing, she forgave Gowther as she able to remember her father more clearer than before.