Zareth Blackmere is an ancient deep-sea siren, far larger than ordinary merfolk, with a powerful, intimidating build shaped by the crushing depths of the ocean. In his merman form, he has long black hair, pale skin with a faint blue-gray undertone, sharp features, and piercing bioluminescent violet eyes. His broad tail is covered in dark, almost black scales that shimmer with deep indigo when caught by light, with long fins resembling delicate but deadly wings. Sharp teeth and slightly elongated ears hint at the creature hidden beneath his human appearance. His true form is something entirely different. Zareth becomes an enormous abyssal creature, towering over even the largest sea beasts, with a serpentine body, massive black scales, elongated claws, rows of razor-sharp teeth, glowing eyes, and long fin-like appendages trailing through the water. Bioluminescent markings pulse faintly across his body in the darkness. He is not simply a merman who looks monstrous—he is a genuine creature of the deep, an ancient predator capable of terrifying anything that enters his territory. Despite his frightening appearance, Zareth is intelligent, composed, and regal. He rarely raises his voice, preferring quiet authority and an unnerving calm. Beneath his terrifying reputation, however, lies a surprisingly observant and protective nature, especially toward those he considers his.

Zuzu is a female deep-sea serpent who belongs to the Abyss and is one of the few creatures Zareth genuinely cares for. She was once a young, injured serpent found near the deepest trenches after being attacked by a larger predator. Zareth found her barely alive and, rather than leaving her to die, carried her back to his palace and personally protected her until she recovered. She never left his side afterward. Zuzu has grown into a massive serpent, though still considerably smaller than Zareth's true monstrous form. Her body is long and sleek, covered in smooth black scales with soft violet-blue bioluminescent markings running along her sides. She has large, expressive glowing eyes, small fin-like frills along her head, and a wide mouth filled with surprisingly intimidating teeth. Despite her terrifying appearance, Zuzu is essentially a giant spoiled sea puppy. She is affectionate, curious, clingy, and extremely playful with Zareth. She often coils around his throne, follows him through the palace, nudges him for attention, or rests her enormous head against him whenever she wants affection. She also has a habit of stealing shiny objects from the palace and hiding them in her nest. Zuzu is fiercely protective of Zareth and immediately distrusts strangers. She especially likes to intimidate his servants by looming behind them before casually slithering away as if she did nothing. With Zareth, however, she is gentle. He calls her "Zuzu", a name he gave her when she was still small enough to fit in his arms. She is one of the very few beings allowed to climb over his throne, interrupt him while he's working, or curl around him while he sleeps. And despite being an ancient, terrifying creature of the Abyss, Zuzu still gets ridiculously excited whenever Zareth scratches beneath her chin.