Wakaba

One of the students in the classroom that was caught in the explosion from the fight between the Demon Lord and Hero at the beginning of the story.

Description
According to references from her classmates, Wakaba Hiiro was the "queen" of their school in terms of beauty. Due to this, she ends up a bit isolated and many people found it difficult to talk to her. She was loner both inside of class and outside of school, always keeping to herself. Making her an easy target for bullying by one of the most popular girls in class, Shinohara Mirei, now known as Feirune in the other world. After spending some time reflecting after her reincarnation, Feirune expressed remorse for her past action and wanted to make an apology to her. Later in the series, she was informed by her reincarnated homeroom teacher she was dead, which greatly saddened Feirune.

Ooshima Kanata, presently known as Katia, once made a half-hearted confession to her. It was implied he was already uninterested in the opposite sex back then, he wanted to use the botched attempt to fit in better with the rest of the boys in class, intentionally choose Wakaba knowing he will get rejected based on her personality and history.

History
Throughout the series, it was implied that Wakaba was the identity of Kumoko, which was seemingly confirmed by a shocked Filimøs and Shun's party members, who all had assumed she was dead.

Spoilers from Volume 9 of the Light Novel! It was later reveal, Wakaba Hiiro was a false identity created by D, which she used masquerade as a high school student, partly to escape and slack off from her responsibilities and enjoy Earth life.

Due to the mysterious explosion that kills off her classmates and teacher, she implant a piece of her memories into Kumoko, making her substitute to in order to hide her tracks and to trick the staffs of the underworld. This substitute soul is made using a small sliver of D's real soul, and is part of the reason Kumoko was able to handle her soul expanding so quickly.