Eris Boreas Greyrat
She is Rudeus' older cousin and also his student. When Rudeus was around seven years old, he taught her magic along with math and reading. She is a violent, strong tomboy and merciless towards anyone who holds negative emotions and intentions against Rudeus. Spoiler When Eris left Rudeus after their first time together, she left her panties and stole a pair of Rudeus' underwear. The current whereabouts of her panties is not known. She leaves him in order to train in the ways of the sword in order to defeat Orsted and eventually reaches Sword King rank. Due to her abrupt disappearance, Rudeus misunderstood her intentions and thought she left him because she didn't like him, thus was the cause for his erectile dysfunction. Spoiler She later marries Rudeus and gives birth to their son Ars. She is one of the three goddesses Rudeus worships in his religion. A wooden sword that serves as a relic for the Goddess of War is enshrined in the basement. (Source: Mushoku Tensei Wikia)