χ

In Amber Era 1586 (around 30 years before Souma enrolled in Academia), he was a human boy confined in the basement below Academia's construction establishment by Dr. Gesellmann. He was named "χ", being the initial test subject for the first "Extreme Transmutation" experiment. His true identity is Gimel Lordbane.
After a long period in confinement, when Gesellmann finally showed up in person, he managed to time his escape. Furthermore, he also engraved a seal on Gesellmann's back as a contract to be his lifetime servant.
Gimel's Will is accompanied by Cecilia's Dying Will, a Vampire who was confined with him, escaped with him, and was burned away under the sunlight.