Bondrewd

Bondrewd is a legendary white whistle delver who has made many great discoveries about the secrets of the Abyss. A researcher fascinated by the Abyss, he has often resorted to illegal and morally questionable means to get funds for and conduct his research, including selling artifacts illicitly, developing dangerous drugs, and even experimenting on living humans. Though how cruel, he still cared for the kids and people he brutally experimented on.

His methods went against all the ethical principles of delvers, but he was allowed to continue due to his outstanding results. Ten years ago, he did the unthinkable by establishing a large base named the Idofront in the deepest area within the 5th Layer of the Abyss, as well as securing a route to the then-inviolable 6th layer.

During one of his experiments researching the Curse of the Abyss, Bondrewd offered children who wanted to unravel the secrets of the Abyss to come with him to the Ido Front. However, he never told them openly that they were going to be used as test subjects. Among the children were Nanachi and Mitty. As Bondrewd continued testing, more and more children were used up in his experiments until only Nanachi and Mitty were left. On his final test, he used both of them, resulting in Mitty being hit with the Curse of the Abyss, while Nanachi received the Blessing.

(Source: Made in Abyss Wiki)