Yomi is a creature who helps Ark on your battles and a guide to outside world. They always are arguing.
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Yomi is only obeying orders of Dark Gaia, and when him orders Yomi kill Ark, he try, but is prevented by Elle, who saves Ark, Elle kills Yomi, however she dies too. Yomi is a copy of the Light Yomi, who was sealed on the outside world by Dark Gaia.