Setsuna Shikido

About
She is the sister of Kaina Shikido. She is first seen by Kaito Aiba outside of Maizuru academy on a rainy day where she collapses. In return for helping them, they later return the items he gave and promise to train him every Sunday when he wants to. The three of them become friends, but they never tell him what they are up to.

She trains physical skills and is an unbelievably skilled magician capable of powerful spatial magic.

They are from 200 years ago and were actually snooping around the dungeon so they could find a way to resurrect the demon lord. Kaina's goal with this is so he can give Setsuna a new life with a new body free of the effects of the racist beatings she endured 200 years prior as half-elves were seen as less than human. However to do so, they need a "Soul Vessel" for the demon lord, which happens to be Meusel Clasmine.

Using the powerful spatial magic they can change a deeper floor to whatever they want, be it the roof or the demon lord's place of sealing.

Her last moments depend on the ending. She either gets killed alone by Kaito and one of his partner's hands or she fights together with Kaina and dies with him. She only gives up her life when her brother dies. She is glad that Kaina would go to such extent for her, but sees the hypocrisy in his ways. She mentions they could live the life they wanted in this new time.