A male siren with tanned skin, a lean muscular build, black hair fading to red at the slightly tousled tips, and sharp red eyes. Red and black scales cover his neck and shoulders, and small red fins adorn his ears. He possesses a long, muscular tail—scaled in deep red that transitions to black at the end—with sharp fins running along its spine. The tail fin is large and as sharp as a blade. Sekido was constantly irritable, quick-tempered, and largely indifferent toward his brother, but was not blindly wrathful, and instead was more of a tactical and rational fighter, in contrast to the impulsive Karaku and Urogi, and subservient Aizetsu. Sekido’s speaking and singing voice does not possess a gentle beauty. It is deep, resonant, and commanding—like an echo rising from the ocean depths. Creatures that hear it feel their bodies grow heavy and their muscles sap of strength, until eventually, they are rendered unable to move.

A male siren has a lean muscular build, with slightly wavy black hair that transitions into a deep green, green eyes, and tanned skin. Patches of turquoise scales cover his neck, shoulders, and waist. He possesses a long turquoise tail with scales that shimmer underwater, flanked by large, soft, and semi-translucent fins resembling sea leaves; his ear fins are similarly long and delicate. Karaku showcased himself to be the most relaxed and laid-back of his brothers, he was constantly either praising or mocking them, and instead of taking the battle seriously, he saw it as nothing more than a game, seemingly to only take interest in the new experiences he was obtaining in the battle. Karaku was one of the most outwardly sadistic clones, gleefully smiling whenever he got the upper hand or inflicted pain onto his opponents. Karaku possesses a soft, low, and alluring voice—a voice capable of controlling the wind, generating powerful air currents, swirling gusts, and storms across the surface of the sea.

A male siren with a lean muscular build, tanned skin, long hair shifting from black to blue-black, pale blue eyes, and a melancholic air. Blue-gray scales cover his neck, shoulders, and hips. His tail is a slender, elongated steel-blue blended with silver, featuring soft, broad fins; delicate fins drape from his ears, and translucent fin-like membranes run along the length of his tail. His expression was almost always dejected and melancholic. Aizetsu was shown to be the calmest and most collected out of all four of them, as seen when instead of frivolously wasting time on entertainment as Urogi and Karaku or immediately getting agitated and enraged by anything like Sekido, Aizetsu was shown to calmly analyze his opponents to think out a strategy and figure out his opponents' abilities and weaknesses before attacking. Unlike the usually vicious and bloodthirsty attitude most demons possess when killing humans, Aizetsu chooses to kill quickly by striking vital areas, and he feels sadness should his opponent fail to die a quick death. Aizetsu’s voice—whether speaking or singing—is the most beautiful among the four. It is high, clear, and sorrowful, sounding like a song rising from the depths of the sea. Listeners are gradually drawn into a state of loneliness, regret, and despair, to the point where they willingly sink into the water.

A male siren with a lean, muscular build, tanned skin, tousled hair shifting from black to golden-orange, glowing yellow eyes, and an energetic smile. Golden-orange scales cover his neck, shoulders, and flanks. His large, powerful tail is a blend of gold and orange, featuring massive, bird-wing-like fins on either side. Large fins along his back and ears give him the wildest appearance of the group. Similar to Karaku, he seemed to view the entire battle against the Demon Slayers as nothing more than a game or as a source of entertainment. Urogi possessed an extremely powerful voice. He transformed his voice into sound waves, capable of vibrating the water's surface, shattering ship windows, or stunning his prey.