Maiden of the Aqua

 This monster appears in the artwork of Astral Shift.

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Yorishiro of the Aqua is a retrained version of this card.

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This card and Gravekeeper's Priestess are the only two monsters that can treat the field as a Field Spell Card.

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They also both have priestess in their Japanese names.

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This monster is one of the few female monsters whose bust size was not reduced in its TCG artwork.

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In video games, however, the bust size of its 3-D model is smaller compared to her artwork.

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This card is similar in appearance to Sailor Mercury from the show Sailor Moon.  Both have blue hair, are related to water, and look physically similar.

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OCG Rulings

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While this monster is face-up on the field, the field is treated as "Umi".: Maiden of the Aqua

Because there is no effect of "Umi", ATK and DEF do not change.

If a Field Spell Card is activated while this monster is face-up on the field, the field becomes the activated Field Spell Card.

The effect of either "Maiden of the Aqua" or "Gravekeeper's Priestess" only applies while there is no Field Spell Card in either player's Field Zone. Even if there is no Field Spell Card in the Field Zone of the player who controls "Maiden of the Aqua" (or "Gravekeeper's Priestess"), if there is a Field Spell Card in their opponent's Field Zone, its effect is not applied.: Do the effects of "Maiden of the Aqua" and "Gravekeeper's Priestess" apply while there is a face-up Field Spell Card on the opponent's field?

Even if the effect of "Maiden of the Aqua" is applying and treating the field as "Umi", each effect of "The Legendary Fisherman" is applied.: If the effect of "Maiden of the Aqua" is applying, does the effect of "The Legendary Fisherman" apply?

When "Abyss-strom" is activated, "A Legendary Ocean" or "Lemuria, the Forgotten City" can be sent to the Graveyard. Furthermore, when "Abyss-strom" is activated, "Maiden of the Aqua" cannot be sent to the Graveyard. (While the effect of "Maiden of the Aqua" is applying, the field is treated as "Umi: Can "Maiden of the Aqua" or "A Legendary Ocean" be sent to the Graveyard when "Abyss-strom" is activated?

Previously Official Rulings

The effect of "Maiden of the Aqua" is active only when there is no face-up Field Spell Card on the field. When a Field Spell Card is activated after "Maiden of the Aqua" is Summoned, the effect of "Maiden of the Aqua" is negated as long as there is a face-up Field Spell Card. If the only Field Spell Card is face-down, the effect of "Maiden of the Aqua" is active. If there is a face-up Field Spell Card but it is negated by "Imperial Order", the effect of "Maiden of the Aqua" is not active.

If a Field Spell Card is already active, and "Maiden of the Aqua" is Summoned, the existing Field Spell Card continues to function and "Maiden of the Aqua"’s effect is not applied until there is no face-up Field Spell Card.

While "Maiden of the Aqua" is making the field "Umi", you can activate "Tornado Wall" but if "Maiden of the Aqua" is destroyed, "Tornado Wall" is also destroyed.

If "Maiden of the Aqua", "Umi", and "Tornado Wall" are all on your side of the field, and "Umi" is destroyed, "Tornado Wall" is not destroyed because "Maiden of the Aqua" treats the field as "Umi" as soon as "Umi" leaves play.

If you have "Maiden of the Aqua" (with her effect active) and "The Legendary Fisherman" or "Deepsea Warrior" on your side of the field, and "Dark Hole" is activated, "The Legendary Fisherman" and "Deepsea Warrior" will not be destroyed.

If you have "Maiden of the Aqua" and "Tornado Wall" on the field, and "Maiden of the Aqua" is attacked by "Dark Driceratops" and Battle Damage would be inflicted, "Tornado Wall" still prevents the damage during damage calculation, as "Maiden of the Aqua" will remain on the field until the end of the Damage Step.

Mentions in Other Rulings

Levia-Dragon - Daedalus: You can send "A Legendary Ocean" to the Graveyard to activate "Levia-Dragon - Daedalus" effect, but you can't send "Maiden of the Aqua".

Mermaid Knight: "Mermaid Knight" can attack twice if "Maiden of the Aqua" is making the field treated as "Umi".

Mermaid Knight: If there is no "Umi" on the field and "Mermaid Knight" attacks, you can Special Summon "Maiden of the Aqua" to treat the field as Umi, you can attack a second time.

Tornado Wall: "Tornado Wall" may be activated when "Maiden of the Aqua" or "A Legendary OceanKonami's OCG Rulings =
If "Maiden of the Aqua" is face-up on the field, then "Mermaid Knight" can attack twice during the same Battle Phase.: If "Maiden of the Aqua" is face-up on the field, can "Mermaid Knight" attack twice during the same Battle Phase?