Mystic Opal Cookie (미스틱오팔 쿠키, misutig-opal kuki) is a Guest Cookie released in the second part of the A Mermaid's Tale update (v4.10), appearing first as an NPC in A Mermaid's Tale Special Episode. Mystic Opal Cookie is the withdrawn and cryptic sister of Black Pearl Cookie, Crimson Coral Cookie, Aquamarine Cookie, and Gold Citrine Cookie, residing in Tearcrown. Her individual power as a Gem Mermaid is control over the surface's weather.
Born with an opal imbued with the powers of wind and fate, Mystic Opal Cookie is a Gem Mermaid blessed with the love of the Sea Fairy Cookie. After the Sea Foam Rite, Mystic Opal Cookie has not only acquired the ability to control the weather but also the gift to sense the grand course of destiny slightly ahead of time. Maybe this is why she prefers hiding in a deep-sea cave in solitude...?
Mystic Opal Cookie's Appearing Cookies section of A Mermaid's Tale reads as such: Mystic Opal Cookie knows that it doesn't make much sense how she can control the weather when she lives underwater. But she says that's exactly why she likes it. She sometimes will receive visions of the future, but she rarely shares them with anyone. Mystic Opal Cookie almost didn't show up to her own Sea Foam Rite because she doesn't enjoy loud and busy places. Or so they say! While she doesn't enjoy the company of other Cookies, Mystic Opal Cookie will still welcome those who visit her cave. Mystic Opal Cookie's internal name is "cookie0585". Mystic Opal Cookie shares her Korean voice actress, Heo Yea-eun, with Serious Paladin Trainee and Parfaedia Principal.