Oh my, who summoned Mussum?! Thank you! It looks like Mussum never fails to impress with her cosplays seeing as how she made it seem like the real Rider just came straight out of the summoning circle.
Rider, real name, Medusa (yes, like the gorgon) is a heroic spirit from the Fate/Stay series. Despite having the same name and somewhat the same story with a bit of addition, this character seems to be loosely patterned after the Medusa from Greek mythology. In Fate/Stay history, Medusa is a beautiful Earth goddess who was loved and adored by everyone. Athena, who was envious of her beauty, cursed Medusa and so the people’s love for her turned to hate and she was banished. Medusa had sisters who went with her in her banishment and kept her company.
When summoned, she appears in her human form but with her eyes covered to seal her very powerful Mystic Eyes of Petrification. Although her sight is completely sealed, her strong sense of hearing, touch, smell, and magic-detection makes it clear that having her eyes covered is hardly a loss in the battlefield.