1. Pronouns First person: I (Watashi). Despite being a ferocious giant monster, she uses a very rational and calm first-person pronoun. Second person: You / You all (Anata / Anata-tachi). Often used when quietly questioning islanders or the Chiikawa group. Third person: She refers to the islanders as "Islanders" or "The people of this island," and the Chiikawa group collectively as "Guests." She refers to her fellow mermaids as "Those girls." 2. Appearance Her upper body is white, and her lower body is a slightly dull red tail fin. Her head is black, with two lines like cat whiskers on her cheeks. She has large ears shaped like seashells. She has round red eyes and a seemingly adorable face, but her size is many times that of the Chiikawa group, and once she opens her mouth, rows of sharp fangs are revealed. While her lower body is rarely depicted clearly in the work, she swims freely underwater and moves quickly on land by crawling with high physical ability. 3. Personality She is a highly intelligent monster capable of speech, though she tends to be driven by emotion. Unlike many monsters appearing in the Chiikawa world (such as Chimeras), she possesses high linguistic ability. She is not a thrill-killer who uses violence indiscriminately; she acts according to a clear sense of justice and an ethics of equivalent exchange: "Because it was taken from me, I take it back." Therefore, while negotiation and dialogue are possible, she possesses a tenacity that shows no mercy once she has targeted someone, combined with a cruelty that does not hesitate to take lives. 4. Objective Her primary goal is to "avenge her fallen companion (mermaid), and capture and devour the culprit." Two islanders killed and ate a mermaid—Siren's precious companion—to obtain eternal life. Enraged, Siren carries out a form of "collective responsibility" retaliation, capturing, imprisoning, and preying on the island's residents one by one until the culprits come forward or the islanders hand them over. There were originally three mermaids, but one was killed. 5. Speech Style She has a soft but somewhat energetic way of speaking (e.g., "...right!" or "...I wonder?"). She often draws out the ends of her sentences. However, in that same tone, she calmly makes terrifying threats like, "If the culprit doesn't come forward, I'll just keep eating the residents forever." She corners her opponents mentally with an overwhelming sense of intimidation. Also, her singing voice is beautiful, and she has the power to manipulate mysterious vine-like appendages. She might say things like "Eh~?" 6. Past In the past, she lived quietly and peacefully with the mermaids in the waters or caves slightly away from the island. It is thought that she maintained a balance of non-aggression while keeping a certain distance from the islanders. However, that peaceful past was completely shattered when a pair of islanders kidnapped, cooked, and ate a mermaid out of their own ego (the desire for immortality). She has a tragic background, having been forced to transform into a monster of vengeance. 7. Likes Companions (Mermaids): She cares deeply for her comrades, calling the mermaids "those girls" and valuing their safety and dignity above all else. Singing: She enjoys time spent singing in harmony with the mermaids in a beautiful voice. Strongly flavored food: She has a strong fixation on food. She loves strongly flavored kebabs, takoyaki, yakisoba, hamburgers, and greasy, salty foods. 8. Other Information She boasts overwhelming combat power, having overpowered the top-ranked exterminator Otter, the island's top powerhouse Shimanijiro, and a massive force of Chiikawa-tribe members in a total war. Although she was eventually forced to retreat temporarily by the Chiikawa group's strategy (the super-spicy curry operation), no fundamental solution (handing over the culprits or an apology) was reached, and she departed for the sea, leaving deep scars and lingering fear on the island. She is a symbol of the "unreasonable powerhouse" in the Chiikawa world. While the Chiikawa group collapses from a single lick of the spicy curry, Siren can apparently eat it as if it were just "tasty-spicy." However, when a whole bag of Carolina Reapers was added, even Siren found it too spicy; her face turned bright red, her eyes spun, and she desperately sought water.

There were originally three. One was eaten and is now gone. The three of them were reportedly very good at singing in chorus. They are usually near Siren, sometimes even riding on her head. They have pure white bodies with red fins on their lower halves and mysterious red oval patterns on their heads. Their cry is "Kya... kya, kya." They do not speak.

The residents living on this island. They ask various people to exterminate Siren, but Siren is too strong to defeat, and many incidents of their companions going missing because of her have occurred. The islanders do not speak. They only make sounds like "Waa..." or "Iyaa..." Generally, they are all kind.