Mirai is a girl who seemed rather defenseless at the beginning of Crystar. That being said, I think it’s reasonable to assume that many people suspected something was up. Ryuk likely wouldn’t fall for her fake personality, but it wouldn’t matter because she could easily overpower him in a fight. Ryuk really doesn’t stand a chance here since Mirai could transform and end him with a single blow. Her super form took multiple magical girls to stop her and each one of them could wreck Ryuk. Mirai wins.