Deep Sea Dolphin is one of the five Demon Lords created by the Demon God Ruby Eye Shabranigdo, and a major yet unseen antagonist in the Slayers manga and light novel series. She is one of the only two female Demon Lords and the least featured of the five. She never involves herself directly in the plot, preferring to act through her subordinates. Like every demon, her ultimate goal is to destroy the world so that her kind could dwell in the Sea of Chaos, source of all creation.
At the beginning was the Sea of Chaos and its personification, the genderless Lord of Nightmares (referred to as a she in spite of her masculine title). Through her dreams, the Lord of Nightmares created four different Worlds with a Demon God, embodiment of Evil, and a Dragon God, embodiment of Good, in each one of them.
In the Slayers World, the Demon God, Ruby Eye Shabranigdo battled the Dragon God, Flare Dragon Ceifeed during many millennia in a conflict known as the "Shinma War". At some point around 5000 years before the events of the series, Shabranigdo created five Demon Lords: Hellmaster Phibrizzo, Supreme King Dynast Grausherra, Chaos Dragon Garv, Greater Beast Zelas Metallium and Sea Queen Deep Sea Dolphin.
At the same time, Ceifeed created four Dragon Lords: Aqualord Ragradia, Airlord Valwind, Earthlord Rangort and Firelord Vrabazard. Shortly after, the two deities defeated one another, Ceifeed having his body destroyed and his spirit scattered all around the world, and Shabranigdo being divided in seven parts which were sealed into human souls.
The Demon and Dragon Lords established themselves in the world and created subordinates; each Demon Lord spawning two high-ranking demons to serve them, one Priest and one General, except Zelas who created one demon to fulfil both roles, this demon named Xellos is as such the strongest high-ranking demon below the Demon Lords themselves.
The War of the Monster's Fall
Over 1000 years before the start of the series, Hellmaster Phibrizzo staged a war between several human nations. He then worsened the conflict, before swarming the lands with lesser demons. His objective was to gather enough negative emotions to awake the fragment of his master which he had discovered sealed in the archmage Lei Magnus.
He eventually succeeded and the newly awoken "Demon King of the North" took command of the demon armies in a worldwide war against the dragon kind. So began the "War of the Monster’s Fall" (Kôma War in Japanese), perhaps the worst conflict in history.
During this war countless of dragons, demons, humans and other creatures were killed. The five Demon Lords managed to make the knowledge of Holy Magic (the most useful weapon against them) almost disappear from the world, by destroying many of Ceifeed's temples. After that, Dolphin joined Phibrizzo, Dynast and Zelas in the creation of the Demon Barrier: a gigantic force-field which separated a huge portion of the world from the other continents. This barrier cut the world in two and prevented the dragons trapped inside from casting Holy Magic, thus drastically weakening them.
The war ended with no defined winner after the death of the Dragon Lord Ragradia (who was in some ways a divine counterpart to Dolphin herself). The Demon Lord Garv was fused with a human soul and Lei Magnus-Shabranigdo was sealed forever in the everlasting ices of the Kataart Mountains.
After the War, Dolphin retreated into her domain in the Demon Sea. What she did is not known exactly but like the others Demon Lords, she maintained her part of the Demon Barrier and ruled the Demon Kind, while planning to destroy the world.
Deep Sea Dolphin is the Demon Lady of water and ocean, who rules over the Demon Sea.
Like all demons of the series, she is an evil spiritual entity from the Astral Plane of existence, who must project an earthly avatar to manifest on the Physical Plane. Like the Demon Lords, she can shape-shift her projection to make it imitate all life-forms perfectly.
Her human-like aspect is depicted as a young woman of regal stature, with long black (or perhaps navy blue) hair and entirely black eyes, wearing a rich blue robe, jewellery ornate with blue gems, and hair ornaments. Her true demonic form was never revealed but considering her element and domain, it can be guessed that she looks like a gigantic, perhaps dolphin-like, sea-monster.
Like all the other Demon Lords, Deep Sea Dolphin is an immensely powerful being, able among many other things to retreat into the Astral plane, to act on the Physical plane while she remains in the Astral, to teleport, shape-shift and make levitate her earthly avatar (as well as other things and people), to create things, to change the weather and to open a sub-dimensional plane. Being a Demon, she can only cast spells of Black Magic (which draws powers from demonic beings); it can be assumed that she has access to all spells of Black Magic save from those using the powers of her fellow Demon Lords and her creator, but this was never fully explained.
As the Demon Lady of Water, she has complete control over her element, being able to manipulate water at will and to control the sea and the sea-monsters. There are two Black Magic spells that draw her power: the Dolf Strash (a highly powerful spear of energy) and the Dolf Zoke (a dangerous wave of spears shaped from water).
Little is known about Dolphin's personality, but given her and her servant's actions, she appears as someone calculating, who sets events into motion and directs the action of her subordinates; as well as someone very greedy and power hungry, who covets complete control over water and the powers linked to it.
A persistent yet baseless fan-theory state shat she and Zelas Metallium hate each other with staunch passion and that Zelas' antics have driven her insane, but this theory was dismissed by Hajime Kanzaka, the creator of the Slayers series. Judging from the plan she devises in Knight of the Aqualord, she seems perfectly sane and as dangerous as her fellow Demon Lords. Also the only time she appears in person in the novel, she is seen alongside Zelas.
Role in the story
In Knight of the Aqualord
Deep Sea Dolphin is the primary antagonist by proxy in the manga Knight of the Aqualord, which takes place after Hellmaster Phibrizzo's destruction and the ensuing collapse of the Demon Barrier. She is behind the main villains of the story and likely devised their plan of action herself.
In this story, the primary protagonists (the sorceress Lina Inverse and the swordsman Gourry Gabriev) are tricked to gamble with a mysterious and clumsy priestess named Huraker, and end up owing her a huge sum of money. Huraker then directs them outside the former Demon Barrier, to get a legendary treasure and repay her. On their way, they reunite with their friend Princess Amelia of Sailune and befriend a young swordsman named Lyos, before confronting a highly powerful female warrior named Riksfalto who fights them frequently.
It appears that Lyos is the legendary Knight of the Aqualord, wielder of (at least a fraction of) the divine power of the late Aqualord Ragradia. Also, it is eventually revealed that Huraker and Riksfalto are in fact Deep Sea Dolphin's Priest and General. Huraker manipulated their entire quest from behind the scenes to get Lyos to awake his divine power, so that they could steal it for their Lady who would wield both demonic and divine control over the ocean. Huraker brainwashes Lyos after he kills Riksfalto but Lina, who gained the power of Airlord Valwind, manages to save him. The protagonist confront the formidable Demon Priestess, who holds her own against the power of two Dragon Lords, and eventually kill her, foiling Dolphin's goal.
In the novels
Deep Sea Dolphin briefly appears in person alongside Zelas Metallium, after Dynast Grausherra successfully awakes the part of Ruby Eye Shabranigdo dormant within a young mercenary named Luke. As Luke has yet to get fully corrupted by the Demon God, this leads to a war between demons following the fully corrupted Lei Magnus Shabranigdo and those following Luke-Shabranigdo. Dolphin and Zelas appear to Lina Inverse and Gourry Gabriev, being ordered by Luke-Shabranigdo to guide them to the dimensional void where he awaits them for a battle to decide the fate of the world.