Guilty

Guilty (グルティー, Girutī) is Ikki's Master on Death Queen Island. His identity is unknown because he always wears a mask.

Chronology
He teaches Ikki to draw power from hate and to provoke Ikki into hate. He killed Esmeralda and just before he died, he revealed to Ikki that Mitsumasa Kido fathered 100 orphan children (only in the manga) and Ikki is one of them. That provoked Ikki to go in a mission of hate and kill all that's related to Mitsumasa Kido.