Juri took over as referee of the Dark Tournament when Koto moved into commentary. She's apparently some kind of sea-creature demon, though the series doesn't dwell on specifics. What's more interesting is that she's genuinely famous—an idol across both the human and demon worlds, which explains why she gets so much screen time during the tournament arc.
She's best known for her over-the-top commentary and enthusiasm during matches, but fans often remember her for a surprisingly personal moment: near the end of the Dark Tournament, she straight-up complains that she's going to die a virgin. It's played for laughs, but apparently it stuck with her. By the time the Makai Tournament rolls around, Koto makes some knowing comments implying Juri got "what she wanted," and there's even a bonus scene showing her watching the tournament alongside a demon with a mustache—presumably her husband. So her arc gets a weirdly wholesome ending for a character who started as pure comic relief.
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