Not much is known about Glariza as she died before the series started.
3,000 years ago
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 before being massacred by the Archangel, Mael. In her final moments, she only asks to be able to see Gowther one last time.
Glariza is briefly seen when Mael searches through his memories while in his own mindscape, where she is seen crying out for Gowther before Mael kills her.
Not much is known about their relationship, but even in her final moments, Glariza wanted to see Gowther once again, and it's implied that they loved each other very much. Her death at the hands of Mael led Gowther to choose him to replace Meliodas in the Demon Clan as karma for her unjustified death.
- Glariza shares her birthday with the puppet Gowther, possibly by his creator to honor his lost lover's memory.