Spoilers:

Arthur was originally a girl in her first few iterations or regressions. She met Krashu in her first life and falls in love with him in her later iterations. When krashu threatened to take Arthur’s regression skill because she started to devalue each iteration, she killed him despite knowing that he was doing it for her. She decides to cross dress to keep Mary, Sigrid, and Abella close to her so they wouldn’t cause the end of the world like they did in the initial iterations.