Roswaal L Mathers/Relationships

Echidna
Somewhere in the past, while Echidna was alive, she took Roswaal as her apprentice. After her death, Roswaal became obsessed with reviving her and killing Volcanica.

Volcanica
Volcanica is the one that kept Echidna's soul in her grave, which was also the reason Roswaal wanted to kill Volcanica.